update docstring for new isSpoiler param

src/Discord.Net.Core/Entities/Channels/IMessageChannel.cs

@@ -61,6 +61,7 @@ namespace Discord
         /// <param name="isTTS">Whether the message should be read aloud by Discord or not.</param>
         /// <param name="embed">The <see cref="Discord.EmbedType.Rich" /> <see cref="Embed" /> to be sent.</param>
         /// <param name="options">The options to be used when sending the request.</param>
+        /// <param name="isSpoiler">Whether the message attachment should be hidden as a spoiler.</param>
         /// <returns>
         ///     A task that represents an asynchronous send operation for delivering the message. The task result
         ///     contains the sent message.
@@ -91,6 +92,7 @@ namespace Discord
         /// <param name="isTTS">Whether the message should be read aloud by Discord or not.</param>
         /// <param name="embed">The <see cref="Discord.EmbedType.Rich"/> <see cref="Embed"/> to be sent.</param>
         /// <param name="options">The options to be used when sending the request.</param>
+        /// <param name="isSpoiler">Whether the message attachment should be hidden as a spoiler.</param>
         /// <returns>
         ///     A task that represents an asynchronous send operation for delivering the message. The task result
         ///     contains the sent message.