In #15884 we adjusted the plan output to give an explicit command to run
to apply a plan, whereas before this command was just alluded to in the
prose.

Since releasing that, we've got good feedback that it's confusing to
include such instructions when Terraform is running in a workflow
automation tool, because such tools usually abstract away exactly what
commands are run and require users to take different actions to
proceed through the workflow.

To accommodate such environments while retaining helpful messages for
normal CLI usage, here we introduce a new environment variable
TF_IN_AUTOMATION which, when set to a non-empty value, is a hint to
Terraform that it isn't being run in an interactive command shell and
it should thus tone down the "next steps" messaging.

The documentation for this setting is included as part of the "...in
automation" guide since it's not generally useful in other cases. We also
intentionally disclaim comprehensive s...