Add xmldoc mention of valid `OnlineID` values

osu.Game/Database/IHasOnlineID.cs

@@ -8,8 +8,12 @@ namespace osu.Game.Database
     public interface IHasOnlineID
     {
         /// <summary>
-        /// The server-side ID representing this instance, if one exists. -1 denotes a missing ID.
+        /// The server-side ID representing this instance, if one exists. Any value 0 or less denotes a missing ID.
         /// </summary>
+        /// <remarks>
+        /// Generally we use -1 when specifying "missing" in code, but values of 0 are also considered missing as the online source
+        /// is generally a MySQL autoincrement value, which can never be 0.
+        /// </remarks>
         int OnlineID { get; }
     }
 }