[SDL] Improved documentation of Event size hack

Source/OpenTK/Platform/SDL2/Sdl2.cs

@@ -1446,8 +1446,9 @@ namespace OpenTK.Platform.SDL2
 #endif
 
         // Ensure the structure is big enough
-        // This hack is necessary, because until we
-        // map all possible events (see above)s
+        // This hack is necessary to ensure compatibility
+        // with different SDL versions, which might have
+        // different sizeof(SDL_Event).
         [FieldOffset(0)]
         private unsafe fixed byte reserved[128];
     }