Avoid references to "visual" positions in favour of positions in the document
structure. The help text shouldn't assume users can see.
Also, in the Posts screen don't mention specific types of posts and use a more
generic text instead.
Props odysseygate, pansotdev, zakb8.
Fixes#34761.
Built from https://develop.svn.wordpress.org/trunk@37680
git-svn-id: http://core.svn.wordpress.org/trunk@37646 1a063a9b-81f0-0310-95a4-ce76da25c4cd