- removes the `Powered by WordPress, state-of-the-art semantic personal publishing platform.` title attribute
- removes the title attribute from the abbreviations: contrary to a common belief, title attributes on abbreviations are only available to a minority of users, see https://developer.paciellogroup.com/blog/2019/03/short-note-the-abbreviation-appreciation-society/
- changes the term `RSS` to `feed`, as `RSS` is a technical term not all users are supposed to be familiar with
- improves the `widget_meta_poweredby` filter documentation
Props nishitlangaliya, chetan200891, audrasjb.
See #46980, #24766.
Fixes#46978.
git-svn-id: https://develop.svn.wordpress.org/trunk@45728 602fd350-edb4-49c9-b593-d223f7449a82