Add custom fill-paragraph function to respect docstrings

[rrudakov]

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• The commits are consistent with our contribution guidelines.
• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
• You've run M-x checkdoc and fixed any warnings in the code you've written.
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[bbatsov]

I think it'd be a good idea to add a test showing it properly handle comments as well.

[rrudakov]

I think it'd be a good idea to add a test showing it properly handle comments as well.

Done.

[bbatsov]

Thanks!