Docs: clarify @since entries added in 4.6

[johnbillion]

After discussing with @drewapicture, some of the @since documentation added in 4.6 should be corrected, clarified, or converted to @internal entries.

[DrewAPicture]

In retrospect, I'm not actually sure the conversion to get_sites() is even noteworthy enough for internal notes. Let's drop them altogether.

As for the two entries in the wp_get_network(), it's basically redundant. Let's drop the changelog entry for 4.4.0 and just stick with the 4.6.0 entry.

[ocean90]

This needs to be reflected in the handbook first:

If significant changes have been made to a function […] additional @since tags, versions, and descriptions should be added to provide a changelog for that function.

“Significant changes” include but are not limited to:

• […]
• Functions or methods becoming wrappers for new APIs

— ​https://make.wordpress.org/core/handbook/best-practices/inline-documentation-standards/php/#since-section-changelogs

[johnbillion]

In 38201:

Docs: Correct and clarify various @since docs.

Fixes #37562

[johnbillion]

In 38202:

Docs: Correct and clarify various @since docs.

Fixes #37562

Merges [38201] to the 4.6 branch.