docs: Small doc improvement for PermissionOverwrites (#1964)

Just so people know what the possible types are.

src/structures/PermissionOverwrites.js

@@ -23,9 +23,16 @@ class PermissionOverwrites {
      */
     this.id = data.id;
 
+    /**
+     * The type of a permission overwrite. It can be one of:
+     * * member
+     * * role
+     * @typedef {string} OverwriteType
+     */
+
     /**
      * The type of this overwrite
-     * @type {string}
+     * @type {OverwriteType}
      */
     this.type = data.type;