Docs: Further update inline documentation for the `wpdb` class per the documentation standards.

Restore some previous edits from [46696], [47060], [47122], [47230] that were accidentally reverted in [47740].

See #49572.
Built from https://develop.svn.wordpress.org/trunk@47749


git-svn-id: https://core.svn.wordpress.org/trunk@47525 1a063a9b-81f0-0310-95a4-ce76da25c4cd