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.

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Improve `wp.media` documentation.

Props ericlewis.
See #28459.

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In `get_page_children()`, only check `$page->ancestors` once to avoid duplicates when the function recurses. Adds an argument, `$ancestors`.

Fixes #18962.

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Twenty Fifteen: Our next-previous labels were reversed.

Props iamtakashi, fixes #30265.


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