Clarify return values for get_preferred_from_update_core() and get_core_updates().

props SergeyBiryukov.
fixes #20251.

Built from https://develop.svn.wordpress.org/trunk@27264


git-svn-id: https://core.svn.wordpress.org/trunk@27121 1a063a9b-81f0-0310-95a4-ce76da25c4cd