Inline documentation for hooks in wp-includes/widgets.php.

Props eliorivero and Nessworthy for the initial patches. Props kpdesign.
Fixes #27129.

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git-svn-id: http://core.svn.wordpress.org/trunk@27701 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Re-instantiate the `MediaElementPlayer` instance when switching mime types in playlists.

Fixes #27608.


Built from https://develop.svn.wordpress.org/trunk@27871


git-svn-id: http://core.svn.wordpress.org/trunk@27702 1a063a9b-81f0-0310-95a4-ce76da25c4cd