Use wp_cache_add() instead of wp_cache_set() when we don't want an unconditional set.

props jeremyfelt.
see #27003.

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Inline documentation for hooks in wp-includes/category-template.php.

Props kpdesign for the cleanup.
Fixes #27432.

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Inline documentation for hooks in wp-admin/includes/post.php.

Also moves the primary PHPDoc for the `edit_posts_per_page` hook to wp-admin/includes/post.php.

Props kpdesign for some cleanup.
Fixes #27431.

Built from https://develop.svn.wordpress.org/trunk@27667


git-svn-id: http://core.svn.wordpress.org/trunk@27510 1a063a9b-81f0-0310-95a4-ce76da25c4cd