Add documentation for IsDefinedMouseAction too

Author: h3xds1nz

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Command/MouseActionConverter.cs

@@ -110,7 +110,11 @@ public override object ConvertTo(ITypeDescriptorContext context, CultureInfo cul
             };
         }
 
-        // Helper like Enum.IsDefined, for MouseAction.
+        /// <summary>
+        /// Helper function similar to <see cref="Enum.IsDefined{MouseAction}(MouseAction)"/>, just lighter and faster.
+        /// </summary>
+        /// <param name="mouseAction">The value to test against.</param>
+        /// <returns><see langword="true"/> if <paramref name="mouseAction"/> falls in enumeration range, <see langword="false"/> otherwise.</returns>
         internal static bool IsDefinedMouseAction(MouseAction mouseAction)
         {
             return mouseAction >= MouseAction.None && mouseAction <= MouseAction.MiddleDoubleClick;