Accessibility: Remove title attributes from the Meta widget.

- 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.

Built from https://develop.svn.wordpress.org/trunk@45728


git-svn-id: http://core.svn.wordpress.org/trunk@45539 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-45727';
+$wp_version = '5.3-alpha-45728';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.