Docs: Add a missing summary and `@since` version to the DocBlock for `_maybe_update_core()`.

Introduced in [10583].

See #32246.

Built from https://develop.svn.wordpress.org/trunk@35985


git-svn-id: https://core.svn.wordpress.org/trunk@35950 1a063a9b-81f0-0310-95a4-ce76da25c4cd