Document invariant

see df4aa7974

src/BizHawk.Client.Common/display/IInputDisplayGenerator.cs

@@ -13,6 +13,7 @@ namespace BizHawk.Client.Common
 	/// </summary>
 	public class Bk2InputDisplayGenerator : IInputDisplayGenerator
 	{
+		/// <remarks>either <c>Range</c> or <c>Mnemonic</c> is always non-null</remarks>
 		private readonly IReadOnlyList<(string Name, AxisSpec? Range, char? Mnemonic)> _cachedInputSpecs;
 
 		private readonly IController _source;