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

Props iamtakashi, fixes #30252.



git-svn-id: https://develop.svn.wordpress.org/trunk@30243 602fd350-edb4-49c9-b593-d223f7449a82
Improve `wp.Uploader` documentation.

Props ericlewis.
See #30260.


git-svn-id: https://develop.svn.wordpress.org/trunk@30244 602fd350-edb4-49c9-b593-d223f7449a82
Improve `wp.media` documentation.

Props ericlewis.
See #28459.


git-svn-id: https://develop.svn.wordpress.org/trunk@30245 602fd350-edb4-49c9-b593-d223f7449a82
In `get_page_children()`, only check `$page->ancestors` once to avoid duplicates when the function recurses. Adds an argument, `$ancestors`.

Fixes #18962.


git-svn-id: https://develop.svn.wordpress.org/trunk@30246 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: Our next-previous labels were reversed.

Props iamtakashi, fixes #30265.



git-svn-id: https://develop.svn.wordpress.org/trunk@30247 602fd350-edb4-49c9-b593-d223f7449a82