Docs: Update the DocBlock for the `wp_media_attach_action` hook per the documentation standards.

Rename the `$att_id` variable to `$attachment_id` for clarity.

Follow-up to [47909].

See #49614.
Built from https://develop.svn.wordpress.org/trunk@47913


git-svn-id: https://core.svn.wordpress.org/trunk@47687 1a063a9b-81f0-0310-95a4-ce76da25c4cd