define Command.args for schemamigration

[eric@…]

Currently, "./manage.py help schemamigration" shows a generic usage string:

Usage: manage.py schemamigration [options]

It would be really great for this to look closer to the usage string shown when if you enter "./manage.py schemamigration":

Usage: ./manage.py schemamigration appname migrationname [--empty] [--initial] [--auto] [--add-model ModelName] [--add-field ModelName.field_name] [--stdout]

At a minimum setting Command.args = "<appname migrationname>" would help newbies realize that you can actually pass a custom migration name. (This is not mentioned in the introductory documentation, and the output of "./migrate.py help schemamigration" does not mention args at all.)