Add note about `Width` requirement in `UserGridPanel`

osu.Game/Users/UserGridPanel.cs

@@ -10,6 +10,10 @@
 
 namespace osu.Game.Users
 {
+    /// <summary>
+    /// A user "card", commonly used in a grid layout or in popovers.
+    /// Comes with a preset height, but width must be specified.
+    /// </summary>
     public partial class UserGridPanel : ExtendedUserPanel
     {
         private const int margin = 10;