Docs: Add a `@since` entry for the `$hook_extra` parameter added to...

Docs: Add a `@since` entry for the `$hook_extra` parameter added to `WP_Upgrader::download_package()`.

Follow-up to [48399].

See #49686.
Built from https://develop.svn.wordpress.org/trunk@48449


git-svn-id: https://core.svn.wordpress.org/trunk@48218 1a063a9b-81f0-0310-95a4-ce76da25c4cd