Add arguments to the wp_dropdown_cats filter, and docs.

props olivM.
fixes #24103.

Built from https://develop.svn.wordpress.org/trunk@27065


git-svn-id: http://core.svn.wordpress.org/trunk@26938 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Ensure the last separator is removed from the user and network admin menus.

props johnjamesjacoby.
fixes #25425.

Built from https://develop.svn.wordpress.org/trunk@27066


git-svn-id: http://core.svn.wordpress.org/trunk@26939 1a063a9b-81f0-0310-95a4-ce76da25c4cd