Clarify requirement of keys for a multi_cached function

[padraic-shafer]

Clarify requirement of keys for a multi_cached function

Updated the docstring of multi_cached decorator to clarify that the keys returned by @multicached cached_dict_func(request_keys=..., keys_from_attr="request_keys", ...) should be the same keys that are passed as request_keys.

Are there changes in behavior for the user?

No. Just documenting the existing behavior.

Related issue number

This fixes a misinterpretation of the expected behavior that was introduced in #637.

Checklist

• [N/A ] I think the code is well written
• [N/A ] Unit tests for the changes exist
• [X ] Documentation reflects the changes
• [N/A ] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.