Improve Message's ID attribute documentation (#1450)

Remove the implication that a Message object's ID is unique only to the channel it was sent on
Message ID's are snowflakes, and as stated in Discord's API documentation, globally unique throughout Discord

src/structures/Message.js

@@ -33,7 +33,7 @@ class Message {
 
   setup(data) { // eslint-disable-line complexity
     /**
-     * The ID of the message (unique in the channel it was sent)
+     * The ID of the message
      * @type {Snowflake}
      */
     this.id = data.id;