Fixed a grammar mistake.

Added a paragraph explaining the use case for outer group submodules.

src/Discord.Net.Commands/Attributes/GroupAttribute.cs

@@ -14,7 +14,8 @@ namespace Discord.Commands
         public string Prefix { get; }
 
         /// <summary>
-        ///     Uses this module as parent instead. Allows creation of group modules outside of parent class.
+        ///     Uses this module as the parent.
+        ///     <para>Useful when group modules are large and nesting them within the parent class becomes difficult to maintain.</para>
         /// </summary>
         public Type ParentModule { get; set; }