[feature/migrations] Comments for the return in the custom functions

PHPBB3-11318

phpBB/includes/db/migration/data/3_0_11_rc1.php

@@ -83,6 +83,7 @@ class phpbb_db_migration_data_3_0_11_rc1 extends phpbb_db_migration
 				WHERE ' . $this->db->sql_in_set('msg_id', $delete_pms);
 			$this->sql_query($sql);
 
+			// Return false to have the Migrator call this function again
 			return false;
 		}
 	}

phpBB/includes/db/migration/data/3_0_7_rc2.php

@@ -62,6 +62,7 @@ class phpbb_db_migration_data_3_0_7_rc2 extends phpbb_db_migration
 			return;
 		}
 
+		// Return the next start, will be sent to $start when this function is called again
 		return $start + $limit;
 	}
 }