Docs: Explicitly document the `rest_revision_query` filter.

The `rest_revision_query` filter in `WP_REST_Revisions_Controller::get_items()` carried a cross-reference comment claiming it was documented in the posts controller. This was incorrect, as the posts controller documents `rest_{$this->post_type}_query` — a dynamic hook that fires only for post types managed by `WP_REST_Posts_Controller`.

`WP_REST_Revisions_Controller` extends `WP_REST_Controller` directly (not the posts controller), so the dynamic hook never fires for revisions. `rest_revision_query` is a standalone hook and has never had its own PHPDoc block.

Follow-up to [43584].

Props apermo, mukesh27, SergeyBiryukov.
See #64224.

git-svn-id: https://develop.svn.wordpress.org/trunk@61998 602fd350-edb4-49c9-b593-d223f7449a82