Document some examples of object types accepted by register_meta()

[DrewAPicture]

As pointed out in a ​contributor note by @janwoostendorp, the current description for the $object_type parameter in register_meta() is a little vague: "Type of object this meta is registered to."

We could do better about documenting a few potential object types that it can accept. Obviously it's not limited to core object types, but giving some examples provides insight into what's expected.

Best recommendation for a patch would be to add an open-ended statement to the description along the lines of "Accepts ... or any other object with an associated meta table."

[sheparddw]

Updated the codex and docblocks.
Let me know if this is what you mean. :)

PR: ​https://github.com/sheparddw/wordpress-develop/pull/1/files
​https://patch-diff.githubusercontent.com/raw/sheparddw/wordpress-develop/pull/1.diff

Codex: ​https://codex.wordpress.org/Function_Reference/register_meta

[kraftbj]

Original patch + PHPCS fixes for updated lines.

[kraftbj]

45464.diff is sheparddw's original patch added to Trac to make it a bit easier to read/apply using the WP patch command. ( ​https://scribu.net/wordpress/contributing-to-wordpress-using-github.html is a handy guide, even though PRs are now accepted).

45464.2.diff fixes a couple of spacing issues in the original patch.

[SergeyBiryukov]

In 47390:

Docs: Improve documentation for various metadata functions.

This adds a more specific description for accepted object types: post, comment, term, user, or any other object type with an associated meta table.

Props sheparddw, kraftbj, DrewAPicture, SergeyBiryukov.
Fixes #45464.

[SergeyBiryukov]

In 48411:

Docs: Synchronize and correct documentation for various metadata functions and filters.

Follow-up to [47390], [47611], [48192], [48402].

See #49572, #43941, #45464.