website: Restore the docs for "terraform 0.12upgrade"

Although this command is removed in Terraform 0.13, our documentation is
for all versions of Terraform that remain in common use and keeping this
documented will be helpful for folks who are still using Terraform 0.11
and planning their upgrades to Terraform 0.12.

Both of the upgrade commands now include notes that they are only
available in their specific major version, along with a link to the
relevant upgrade guide for other background information about the upgrade,
in case the user finds the command documentation first. (The command docs
are, I think, a little more discoverable than the upgrade guides.)

website/docs/commands/0.12upgrade.html.markdown

@@ -0,0 +1,118 @@
+---
+layout: "docs"
+page_title: "Command: 0.12upgrade"
+sidebar_current: "docs-commands-012upgrade"
+description: |-
+  The 0.12upgrade subcommand automatically rewrites existing configurations for Terraform 0.12 compatibility.
+---
+
+# Command: 0.12upgrade
+
+The `terraform 0.12upgrade` command applies several automatic upgrade rules to
+help prepare a module that was written for Terraform v0.11 to be used
+with Terraform v0.12.
+
+-> **This command is available only in Terraform v0.12 releases.** For more information, see [the Terraform v0.12 upgrade guide](https://www.terraform.io/upgrade-guides/0-12.html).
+
+## Usage
+
+Usage: `terraform 0.12upgrade [options] [dir]`
+
+By default, `0.12upgrade` changes configuration files in the current working
+directory. However, you can provide an explicit path to another directory if
+desired, which may be useful for automating migrations of several modules in
+the same repository.
+
+When run with no other options, the command will first explain what it is
+going to do and prompt for confirmation:
+
+```
+$ terraform 0.12upgrade
+
+This command will rewrite the configuration files in the given directory so
+that they use the new syntax features from Terraform v0.12, and will identify
+any constructs that may need to be adjusted for correct operation with
+Terraform v0.12.
+
+We recommend using this command in a clean version control work tree, so that
+you can easily see the proposed changes as a diff against the latest commit.
+If you have uncommitted changes already present, we recommend aborting this
+command and dealing with them before running this command again.
+
+Would you like to upgrade the module in the current directory?
+  Only 'yes' will be accepted to confirm.
+
+  Enter a value: yes
+```
+
+The `0.12upgrade` subcommand requires access to providers used in the
+configuration in order to analyze their resource types, so it's important to
+run `terraform init` first to install these. In some rare cases, a configuration
+that worked in v0.11 may have syntax errors in v0.12, in which case
+`terraform init` will run in a special mode where it installs only enough to
+run the upgrade command, after which you can run `terraform init` again to
+complete initialization.
+
+Many of the rewrite rules are completely automatic, but in some cases the
+tool cannot determine enough information from the configuration alone to make
+a decision, and so it will instead add a comment to the configuration for
+user review. All such comments contain the string `TF-UPGRADE-TODO` to make
+them easy to find.
+
+After upgrading, the configuration will also be reformatted into the standard
+Terraform style and expressions rewritten to use the more-readable v0.12 syntax
+features.
+
+We recommend running this command with a clean version control work tree so
+that you can use VCS tools to review the proposed changes, including any
+`TF-UPGRADE-TODO` comments, and make any revisions required before committing
+the change.
+
+Once upgraded the configuration will no longer be compatible with Terraform
+v0.11 and earlier. When upgrading a shared module that is called from multiple
+configurations, you may need to
+[fix existing configurations to a previous version](/docs/configuration/modules.html#module-versions)
+to allow for a gradual upgrade. If the module is published via
+[a Terraform registry](/docs/registry/), assign a new _major_ version number
+to the upgraded module source to represent the fact that this is a breaking
+change for v0.11 callers. If a module is installed directly from a version
+control system such as Git,
+[use specific revisions](https://www.terraform.io/docs/modules/sources.html#selecting-a-revision)
+to control which version is used by which caller.
+
+The command-line options are all optional. The available options are:
+
+* `-yes` - Skip the initial introduction messages and interactive confirmation.
+  Use this when running the command in batch from a script.
+
+* `-force` - Override the heuristic that attempts to detect if a configuration
+  is already written for v0.12 or later. Some of the transformations made by
+  this command are not idempotent, so re-running against the same module may
+  change the meanings of some expressions in the module.
+
+## Batch Usage
+
+After you've experimented with the `0.12upgrade` command in some confined
+situations, if you have a repository containing multiple modules you may
+wish to batch-upgrade them all and review them together. Recursive upgrades
+are not supported by the tool itself, but if you are on a Unix-style system
+you can achieve this using the `find` command as follows:
+
+```
+find . -name '*.tf' -printf "%h\n" | uniq | xargs -n1 terraform 0.12upgrade -yes
+```
+
+On Mac OS X, the `find` included with the system does not support the `-printf` argument. You can install GNU find using Homebrew in order to use that argument:
+
+```
+brew install findutils
+```
+Once installed, run the above command line using `gfind` instead of `find`.
+
+
+Note that the above includes the `-yes` option to override the interactive
+prompt, so be sure you have a clean work tree before running it.
+
+Because upgrading requires access to the configuration's provider plugins,
+all of the directories must be initialized with `terraform init` prior to
+running the above.

website/docs/commands/0.13upgrade.html.markdown

@@ -12,7 +12,7 @@ The `terraform 0.13upgrade` command updates existing configuration to add an
 explicit `source` attribute for each provider used in a given module. The
 provider source settings are stored in a `required_providers` block.
 
--> This command requires **Terraform v0.13 or later**.
+-> **This command is available only in Terraform v0.13 releases.** For more information, see [the Terraform v0.13 upgrade guide](https://www.terraform.io/upgrade-guides/0-13.html).
 
 ## Usage
 

website/layouts/docs.erb

@@ -270,6 +270,10 @@
           <li<%= sidebar_current("docs-commands-013upgrade") %>>
             <a href="/docs/commands/0.13upgrade.html">0.13upgrade</a>
           </li>
+
+          <li<%= sidebar_current("docs-commands-012upgrade") %>>
+            <a href="/docs/commands/0.12upgrade.html">0.12upgrade</a>
+          </li>
         </ul>
       </li>