command: Remove promise of plan -validate-only from validate docs

We brought forward a new implementation of "terraform validate" that was
originally scheduled for a later release after finding that it would be
simpler than reworking the old implementation for new v0.12 assumptions,
but we didn't yet implement "terraform plan -validate-only" in spite of
it being mentioned in the updated docs for "terraform validate".

For now then, the documentation will make the weaker suggestion of running
"terraform plan" to validate a particular _run_ rather than a particular
_module_, which is the closest thing we have for now. At some point after
v0.12.0 we will evaluate whether a validate-only mode for "terraform plan"
(which could then run without configuring the providers at all) is needed.

command/validate.go

@@ -234,11 +234,6 @@ Usage: terraform validate [options] [dir]
   state. It is thus primarily useful for general verification of reusable
   modules, including correctness of attribute names and value types.
 
-  To verify configuration in the context of a particular run (a particular
-  target workspace, operation variables, etc), use the following command
-  instead:
-      terraform plan -validate-only
-
   It is safe to run this command automatically, for example as a post-save
   check in a text editor or as a test step for a re-usable module in a CI
   system.
@@ -250,6 +245,10 @@ Usage: terraform validate [options] [dir]
 
   If dir is not specified, then the current directory will be used.
 
+  To verify configuration in the context of a particular run (a particular
+  target workspace, operation variables, etc), use the terraform plan
+  subcommand instead, which includes an implied validation check.
+
 Options:
 
   -json        Produce output in a machine-readable JSON format, suitable for