DB migration 000035: Add explainatory comment

Author: Nikos410

resources/migrations/000035_add_song_requests_indices.up.sql

@@ -1,5 +1,7 @@
 BEGIN;
 
+-- This index also acts as a unique constraint to ensure that only one
+-- CURRENTLY_PLAYING and one UP_NEXT request are present per session
 CREATE UNIQUE INDEX song_requests_status_index
     ON song_requests (session_id, status)
     WHERE status in ('CURRENTLY_PLAYING', 'UP_NEXT');