Docs: Add a `@since` note to the `display_post_states` filter to clarify that...

Docs: Add a `@since` note to the `display_post_states` filter to clarify that it is now also applied in the Customizer context.

If any admin functions are used within the filter, their existence should be checked with `function_exists()` before being used.

Follow-up to [47763], [48620].

Props audrasjb, garrett-eclipse, Howdy_McGee, dlh, khag7, SergeyBiryukov.
Merges [48910] to the 5.5 branch.
Fixes #51081.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48677 1a063a9b-81f0-0310-95a4-ce76da25c4cd