[Fn::Sub] Relax documented restrictions on String parameter

[LaikaN57]

Issue #, if available:

aws-cloudformation/cfn-lint#2587

Description of changes:

Clarify the ability to use an Fn::Transform in the String parameter of an Fn::Sub.

Discussion

Looking for feedback on how to better integrate the use of Fn::Transform into the documentation as a whole. I think one way to do this is to relax some of the strict statements around the section I am editing here. If there is a better doc writer out there that wants to help me wordsmith the docs here, I am completely open to that.

I feel that Fn::Transform is a bit more of an advanced CFN tool, and thus we may need to balance detail vs the need for simple documentation for more common features. However, I do strongly believe that the docs should not make strict claims about what is or is not possible in CFN. Meaning, the docs should maintain technical correctness.

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[joshbean]

Closing this issue or pull request in advance of archiving this repo. For more information about the decision to archive this repo (and others in the 'awsdocs' org), see the announcement on the AWS News Blog.