Correct and standardise the meta query documentation

[johnbillion]

The following query classes all support meta_* arguments because they pass their arguments onto WP_Meta_Query, however their documentation is inconsistent, incomplete, and in some cases incorrect.

• WP_Comment_Query
• WP_Query
• WP_Site_Query
• WP_Term_Query
• WP_User_Query

Notably, WP_Query documents a meta_value_num argument which does not exist. This was erroneously introduced in [28887]. meta_value_num is an allowed value for orderby arguments, but is not the name of an argument itself.

Let's standardise the documentation and guide users to WP_Meta_Query for the canonical documentation.

Trac ticket: https://core.trac.wordpress.org/ticket/53467

@johnbillion I resolved the conflicts, but I left the indentation as it was in your previous proposal.

[johnbillion]

In 52226:

Query: Correct and standardise the meta query documentation.

Also improves the formatting of some surrounding documentation.

Props audrasjb, johnbillion

Fixes #53467

https://core.trac.wordpress.org/changeset/52226