Fill out inline documentation for magic methods added to the `Custom_Background` class in [28481], [28521], and [28524].

See #22234 and #28885.

Built from https://develop.svn.wordpress.org/trunk@29149


git-svn-id: http://core.svn.wordpress.org/trunk@28933 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fill out inline documentation for magic methods added to the `Custom_Image_Header` class in [28481], [28521], and [28524].

See #22234 and #28885.

Built from https://develop.svn.wordpress.org/trunk@29150


git-svn-id: http://core.svn.wordpress.org/trunk@28934 1a063a9b-81f0-0310-95a4-ce76da25c4cd