Inline documentation for hooks in wp-includes/taxonomy.php.

Props kpdesign for some minor language tweaks.
Fixes #27505.

Built from https://develop.svn.wordpress.org/trunk@27827


git-svn-id: http://core.svn.wordpress.org/trunk@27661 1a063a9b-81f0-0310-95a4-ce76da25c4cd