Inline documentation improvements for `Walker_Category` and `Walker_CategoryDropdown`.

Also covers previously undocumented parameters.

See #23406, #19249.

Built from https://develop.svn.wordpress.org/trunk@25641


git-svn-id: http://core.svn.wordpress.org/trunk@25558 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Inline documentation for `Walker_Category_Checklist`.

See #23406.

Built from https://develop.svn.wordpress.org/trunk@25642


git-svn-id: http://core.svn.wordpress.org/trunk@25559 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Merge branch 'master' of https://github.com/WordPress/WordPress