Enhance Style/FormatString's autocorrection

[koic]

This PR enhances Style/FormatString's autocorrection when using known conversion methods whose return value is not an array.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Might be a good idea to document the autocorrection logic, so users know what to expect.

[koic]

Sure! I've updated the document.