Additional documentation included.

src/Discord.Net.Core/Entities/Messages/IUserMessage.cs

@@ -58,14 +58,17 @@ namespace Discord
         Task UnpinAsync(RequestOptions options = null);
 
         /// <summary>
-        ///     Publish (crosspost) this message.
+        ///     Publishes (crossposts) this message.
         /// </summary>
         /// <param name="options">The options to be used when sending the request.</param>
         /// <returns>
         ///     A task that represents the asynchronous operation for publishing this message.
         /// </returns>
         /// <remarks>
-        ///     Publishing (crossposting) is only available in news channels.
+        ///     <note type="warning">
+        ///         This call will throw an <see cref="InvalidOperationException"/> if attempted in a non-news channel.
+        ///     </note>
+        ///     This method will publish (crosspost) the message. Please note, publishing (crossposting), is only available in news channels.
         /// </remarks>
         Task CrosspostAsync(RequestOptions options = null);
 

src/Discord.Net.Rest/Entities/Messages/RestUserMessage.cs

@@ -149,8 +149,10 @@ namespace Discord.Rest
             => MentionUtils.Resolve(this, 0, userHandling, channelHandling, roleHandling, everyoneHandling, emojiHandling);
 
         /// <inheritdoc />
+        /// <exception cref="InvalidOperationException">This operation may only be called on a <see cref="RestNewsChannel"/> channel.</exception>
         public async Task CrosspostAsync(RequestOptions options = null)
         {
+            // Validate that the channel is of type RestNewsChannel
             if (!(Channel is RestNewsChannel))
             {
                 throw new InvalidOperationException("Publishing (crossposting) is only valid in news channels.");

src/Discord.Net.WebSocket/Entities/Messages/SocketUserMessage.cs

@@ -149,8 +149,10 @@ namespace Discord.WebSocket
             => MentionUtils.Resolve(this, 0, userHandling, channelHandling, roleHandling, everyoneHandling, emojiHandling);
 
         /// <inheritdoc />
+        /// <exception cref="InvalidOperationException">This operation may only be called on a <see cref="SocketNewsChannel"/> channel.</exception>
         public async Task CrosspostAsync(RequestOptions options = null)
         {
+            // Validate that the channel is of type SocketNewsChannel
             if (!(Channel is SocketNewsChannel))
             {
                 throw new InvalidOperationException("Publishing (crossposting) is only valid in news channels.");