docs: Add/normalize .toString() docs on all classes (#2042)

* docs: Add/normalize .toString() examples on all classes

* docs: Remove exclamation point on ClientApplication#toString example

* docs: Normalize .toString() descriptions on all classes

* Use "returns" instead of "concatenates"

src/structures/GroupDMChannel.js

@@ -203,14 +203,12 @@ class GroupDMChannel extends Channel {
   }
 
   /**
-   * When concatenated with a string, this automatically concatenates the channel's name instead of the Channel object.
+   * When concatenated with a string, this automatically returns the channel's name instead of the
+   * GroupDMChannel object.
    * @returns {string}
    * @example
    * // Logs: Hello from My Group DM!
    * console.log(`Hello from ${channel}!`);
-   * @example
-   * // Logs: Hello from My Group DM!
-   * console.log(`Hello from ' + channel + '!');
    */
   toString() {
     return this.name;