Fix incomplete inline documentation for the `wp_header_image_attachment_metadata` filter.

See #26869.


git-svn-id: https://develop.svn.wordpress.org/trunk@28375 602fd350-edb4-49c9-b593-d223f7449a82
Clarify documentation for *what* gets "automatically fired" when the `do_mu_upgrade` filter evaluates to true in Multisite.

See #26869.


git-svn-id: https://develop.svn.wordpress.org/trunk@28376 602fd350-edb4-49c9-b593-d223f7449a82