Improve inline documentation for `wpdb::has_cap()`.

Adds a changelog entry for the addition of the 'utf8mb4' feature, accepted values for `$db_cap`, and a return description.

Props kpdesign.
See #21212.


git-svn-id: https://develop.svn.wordpress.org/trunk@30768 602fd350-edb4-49c9-b593-d223f7449a82