add documentation for CCK fieldgroups to usage.txt

There are quite a few support requests from people that can't figure out how to make field groups work. It's documented in the issue queue, but I think it wouldn't harm to add a bit of documentation on the subject in usage.txt somewhere in the CCK description. For example:

When your field is part of a CCK fieldgroup you'll have to prepend your prepopulate url with [group_groupname] for example:
edit[group_myfieldgroup][field_nodereference][0][nid][nid]=[nid: #]