Twenty Fifteen: broader selection of avatars that should catch any plugins adding avatars that use the avatar class.

Props iamtakashi, fixes #30541.


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git-svn-id: http://core.svn.wordpress.org/trunk@30680 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve inline documentation for four methods in `WP_Widget`: `get_field_id()`, `display_callback()`, `update_callback()`, and `form_callback()`.

Props jazzs3quence for the initial patch.
See #30315.

Built from https://develop.svn.wordpress.org/trunk@30691


git-svn-id: http://core.svn.wordpress.org/trunk@30681 1a063a9b-81f0-0310-95a4-ce76da25c4cd