Docs: Add a missing summary, `@since` version, and parameter descriptions to...

Docs: Add a missing summary, `@since` version, and parameter descriptions to the DocBlock for `_register_widget_update_callback()`.

Introduced in [10798].

See #32246.

Built from https://develop.svn.wordpress.org/trunk@35989


git-svn-id: https://core.svn.wordpress.org/trunk@35954 1a063a9b-81f0-0310-95a4-ce76da25c4cd