Updates to post.php help text for posts and pages, including a new section on...

Updates to post.php help text for posts and pages, including a new section on inserting media. props DrewAPicture, Ipstenu. see #22451.



git-svn-id: https://core.svn.wordpress.org/trunk@22813 1a063a9b-81f0-0310-95a4-ce76da25c4cd