Add docs to `OpposingDirPolicy` enum

see #2752

src/BizHawk.Client.Common/config/ConfigEnums.cs

@@ -30,10 +30,17 @@
 		N64Tas = 4
 	}
 
+	/// <summary>
+	/// indicates one of the possible approaches for handling simultaneous opposing cardinal directions (SOCD)
+	/// e.g. <c>DPad Left</c>+<c>DPad Right</c>, which may not have been possible with original gamepad hardware
+	/// </summary>
 	public enum OpposingDirPolicy
 	{
+		/// <summary>if both directions are pressed, only the most recently pressed (of the pair) will be sent</summary>
 		Priority = 0,
+		/// <summary>if both directions are pressed, they will "cancel out" and neither will be sent</summary>
 		Forbid = 1,
+		/// <summary>both directions will be sent when both are pressed</summary>
 		Allow = 2,
 	}
 }