Twenty Fifteen: "detach" widgets from the header in mobile views.

Props iamtakashi, fixes #30252.


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Improve `wp.Uploader` documentation.

Props ericlewis.
See #30260.

Built from https://develop.svn.wordpress.org/trunk@30244


git-svn-id: http://core.svn.wordpress.org/trunk@30244 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve `wp.media` documentation.

Props ericlewis.
See #28459.

Built from https://develop.svn.wordpress.org/trunk@30245


git-svn-id: http://core.svn.wordpress.org/trunk@30245 1a063a9b-81f0-0310-95a4-ce76da25c4cd