Inline documentation cleanup for 4.0 audit.

Added in [28652]:
* Various phpDoc improvements for `wp_embed_handler_youtube()`
* Adds `@see` reference to `wp_embed_handler_youtube()` in 'wp_embed_handler_youtube' filter docs

Added in [29029]:
* s/post_id/post ID in `@return` description for `attachment_url_to_postid()`

See #28885.

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git-svn-id: http://core.svn.wordpress.org/trunk@28948 1a063a9b-81f0-0310-95a4-ce76da25c4cd