Improve and clarify inline commenting inside the `check_comment()` function.

Adds logical explanations of what some of the various comment checks are checking for, as well as some general cleanup and syntax fixes.

Fixes #29734.

Built from https://develop.svn.wordpress.org/trunk@29763


git-svn-id: http://core.svn.wordpress.org/trunk@29535 1a063a9b-81f0-0310-95a4-ce76da25c4cd