[Fix #5713] Be more explicit about how Style/CommentAnnotation works

[dvandersluis]

The consensus in the issue is that the cop is working as expected, but the documentation does not explain well enough. This change adds an explicit test for comment blocks, as well as a documentation note.

Closes #5713.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks!