Docs: Improve comments in some `wp-admin` files per the documentation standards.

Follow-up to [47084].

Props passoniate, apedog.
Fixes #49223, #49227.
Built from https://develop.svn.wordpress.org/trunk@47119


git-svn-id: https://core.svn.wordpress.org/trunk@46919 1a063a9b-81f0-0310-95a4-ce76da25c4cd