Options, Meta APIs: Update the docs around default metadata values.

Add a missing @since value for 5.5 and potential default filter names.

Props johnbillion.
Reviewed by whyisjake, SergeyBiryukov.
Merges [48777] and [48778] to the 5.5 branch.
Fixes #50891.
Built from https://develop.svn.wordpress.org/branches/5.5@48779


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48541 1a063a9b-81f0-0310-95a4-ce76da25c4cd