Improve documentation in the `check_comments()` docblock.

* Removes two duplicate sentences from the long description
* Tightens up phrasing elsewhere in the long description for specific checks
* Clarifies the parameter and return descriptions, and adds periods

See #29734.

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git-svn-id: http://core.svn.wordpress.org/trunk@29534 1a063a9b-81f0-0310-95a4-ce76da25c4cd