PHPDoc fixes and additions. fixes #24616.

git-svn-id: http://core.svn.wordpress.org/trunk@24490 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Correct return value for get_role() and WP_Roles::get_role(). props tivnet. fixes #24607.

git-svn-id: http://core.svn.wordpress.org/trunk@24491 1a063a9b-81f0-0310-95a4-ce76da25c4cd