Translate custom header customize control strings. Remove `final` added before beta to see what breakage we might cause by refactoring this class.

see #21785. fixes #27738.

Built from https://develop.svn.wordpress.org/trunk@28082


git-svn-id: http://core.svn.wordpress.org/trunk@27913 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Priority fixes for various existing hook documentation.

Props kpdesign.
See #26869

Built from https://develop.svn.wordpress.org/trunk@28083


git-svn-id: http://core.svn.wordpress.org/trunk@27914 1a063a9b-81f0-0310-95a4-ce76da25c4cd