Multisite: Document all return types in get_networks().

Adds more complete documentation to get_networks() and the corresponding methods in WP_Network_Query.

Fixes #42260.

Built from https://develop.svn.wordpress.org/trunk@41918


git-svn-id: https://core.svn.wordpress.org/trunk@41752 1a063a9b-81f0-0310-95a4-ce76da25c4cd