Update documentation of SteamGroup#fetch

See koraktor/steam-condenser#234

lib/steam/community/steam_group.rb

@@ -1,7 +1,7 @@
 # This code is free software; you can redistribute it and/or modify it under
 # the terms of the new BSD License.
 #
-# Copyright (c) 2008-2013, Sebastian Staudt
+# Copyright (c) 2008-2015, Sebastian Staudt
 
 require 'errors/steam_condenser_error'
 require 'steam/community/cacheable'
@@ -96,7 +96,10 @@ def base_url
     end
   end
 
-  # Loads the members of this group
+  # Loads information about and members of this group
+  #
+  # This includes the ID, name, headline, summary of the group as well as
+  # avatar and custom URLs.
   #
   # This might take several HTTP requests as the Steam Community splits this
   # data over several XML documents if the group has lots of members.