Added ChannelTypes documentation

src/Discord.Net/ChannelTypes.cs

@@ -2,7 +2,9 @@
 {
 	public static class ChannelTypes
 	{
+		/// <summary> A text-only channel. </summary>
 		public const string Text = "text";
+		/// <summary> A voice-only channel. </summary>
 		public const string Voice = "voice";
 	}
 }