website: More words about "terraform fmt"

We've historically made statements like this in response to requests for
more customization to the "terraform fmt" behavior, but the documentation
itself was somewhat vague about the intended goals of this command.

This is an attempt to be more explicit that consistency between codebases
is the primary goal of this command, and that the examples in the
Terraform documentation are our main guide for what is "idiomatic style"
when adding additional rules over time.

Nothing here is intended to be new policy, but instead as codifying
positions we've taken elsewhere in the past in the hope of allowing users
to decide how (and whether) they wish to make use of this tool.

website/docs/commands/fmt.html.markdown

@@ -22,6 +22,28 @@ after upgrading Terraform we recommend to proactively run `terraform fmt`
 on your modules along with any other changes you are making to adopt the new
 version.
 
+We don't consider new formatting rules in `terraform fmt` to be a breaking
+change in new versions of Terraform, but we do aim to minimize changes for
+configurations that are already following the style examples shown in the
+Terraform documentation. When adding new formatting rules, they will usually
+aim to apply more of the rules already shown in the configuration examples
+in the documentation, and so we recommend following the documented style even
+for decisions that `terraform fmt` doesn't yet apply automatically.
+
+Formatting decisions are always subjective and so you might disagree with the
+decisions that `terraform fmt` makes. This command is intentionally opinionated
+and has no customization options because its primary goal is to encourage
+consistency of style between different Terraform codebases, even though the
+chosen style can never be be everyone's favorite.
+
+We recommend that you follow the style conventions applied by `terraform fmt`
+when writing Terraform modules, but if you find the results particularly
+objectionable then you may choose not to use this command, and possibly choose
+to use a third-party formatting tool instead. If you choose to use a
+third-party tool then you should also run it on files that are generated
+automatically by Terraform, to get consistency between your hand-written files
+and the generated files.
+
 ## Usage
 
 Usage: `terraform fmt [options] [DIR]`