website: Revise the "Modules" configuration docs section

This adopts a more guide-like writing style, similar to what prior commits
have done to some other subsections of this section.

Since we already have a whole top-level section devoted to modules, there
is no need for full coverage of all of their features here. Instead, this
section focuses on an an initial introduction to what modules are and
the basics of their usage within the Terraform language. We then link
to the main modules section for the full details.

website/docs/configuration/modules.html.md

@@ -3,46 +3,84 @@ layout: "docs"
 page_title: "Configuring Modules"
 sidebar_current: "docs-config-modules"
 description: |-
-  Modules are used in Terraform to modularize and encapsulate groups of resources in your infrastructure. For more information on modules, see the dedicated modules section.
+  Modules allow multiple resources to be grouped together and encapsulated.
 ---
 
-# Module Configuration
+# Modules
 
-Modules are used in Terraform to modularize and encapsulate groups of
-resources in your infrastructure. For more information on modules, see
-the dedicated
-[modules section](/docs/modules/index.html).
+A _module_ is a container for multiple resources that are used together.
+Every Terraform configuration has at least one module, known as its
+_root module_, which consists of the resources defined in the `.tf` files in
+the main working directory.
 
-This page assumes you're familiar with the
-[configuration syntax](/docs/configuration/syntax.html)
-already.
+A module can call other modules, allowing the suite of resources within the
+child module to be included into the configuration in a concise way. Modules
+can also be called multiple times, either within the same configuration or
+in separate configurations, allowing resource configurations to be packaged
+and re-used.
 
-## Example
+This page describes how to call one module from another. Other pages in this
+section of the documentation describe the different elements that make up
+modules, and there is further information about how modules can be used,
+created, and published in [the dedicated _Modules_ section](/docs/modules/index.html).
+
+## Calling a Child Module
+
+To _call_ a module means to include the contents of that module into the
+configuration with specific values for its
+[input variables](/docs/configuration/variables.html). Modules are called
+from within other modules using `module` blocks:
 
 ```hcl
-module "consul" {
-  source  = "hashicorp/consul/aws"
+module "servers" {
+  source = "./app-cluster"
+
   servers = 5
 }
 ```
 
-## Description
+The label immediately after the `module` keyword is a name that will be used
+to refer to this instance of the module within the calling module. The
+_calling module_ is the one that includes the `module` block shown above.
 
-A `module` block instructs Terraform to create an instance of a module,
-and in turn to instantiate any resources defined within it.
+Within the block body (between `{` and `}`) are the arguments for the module.
+Most of the arguments correspond to [input variables](/docs/configuration/variables.html)
+defined by the module, including the `servers` argument in the above example.
 
-The name given in the block header is used to reference the particular module
-instance from expressions within the calling module, and to refer to the
-module on the command line. It has no meaning outside of a particular
-Terraform configuration.
+The `source` argument is a meta-argument defined and processed by Terraform
+itself. Its value is the path to a local directory containing the module's
+configuration files, or optionally a remote module source that Terraform should
+download and use. For more information on possible values for this argument,
+see [_Module Sources_](/docs/modules/sources.html).
 
-Within the block body is the configuration for the module. All attributes
-within the block must correspond to [variables](/docs/configuration/variables.html)
-within the module, with the exception of the following which Terraform
-treats as special:
+The same source address can be specified in multiple `module` blocks to create
+multiple copies of the resources defined within, possibly with different
+variable values.
 
-* `source` - (Required) A [module source](/docs/modules/sources.html) string
-  specifying the location of the child module source code.
+## Accessing Module Output Values
+
+The resources defined in a module are encapsulated, so the calling module
+cannot access their attributes directly. However, the child module can
+declare [output values](/docs/configuration/outputs.html) to selectively
+export certain values to be accessed by the calling module.
+
+For example, if the `./app-cluster` module referenced in the example above
+exported an output value named `instance_ids` then the calling module
+can reference that result using the expression `module.servers.instance_ids`:
+
+```hcl
+resource "aws_elb" "example" {
+  # ...
+
+  instances = module.servers.instance_ids
+}
+```
+
+## Other Meta-arguments
+
+Along with the `source` meta-argument described above, module blocks have
+some more meta-arguments that have special meaning across all modules,
+described in more detail in other sections:
 
 * `version` - (Optional) A [version constraint](/docs/modules/usage.html#module-versions)
   string that specifies which versions of the referenced module are acceptable.
@@ -55,3 +93,11 @@ treats as special:
   [provider configurations to be passed explicitly to child modules](/docs/modules/usage.html#providers-within-modules).
   If not specified, the child module inherits all of the default (un-aliased)
   provider configurations from the calling module.
+
+In addition to the above, the argument names `count`, `for_each` and
+`lifecycle` are not currently used by Terraform but are reserved for planned
+future features.
+
+Since modules are a complex feature in their own right, further detail
+about how modules can be used, created, and published is included in
+[the dedicated section on modules](/docs/modules/index.html).

website/docs/configuration/variables.html.md

@@ -60,6 +60,23 @@ if no overridden value is set when calling the module. The `default` argument
 requires a literal value and cannot reference other objects in the
 configuration.
 
+## Using Input Variable Values
+
+Within the module that declared a variable, its value can be accessed from
+within [expressions](/docs/configuration/expressions.html) using an expression
+like `var.image_id`, where the name after the period corresponds to the label
+given in the declaration block:
+
+```hcl
+resource "aws_instance" "example" {
+  instance_type = "t2.micro"
+  ami           = var.image_id
+}
+```
+
+The value assigned to a variable can be accessed only from expressions within
+the module where it was declared.
+
 ## Type Constraints
 
 The `type` argument in a `variable` block allows you to restrict the type of