Improve drush command help

The drush fserver-distro command help text does not tell the user how to properly use the command at the moment. It should explain at least that the usage pattern is drush fserver-distro [distro_name] --version [version-tag].

It might also be worth considering a different usage pattern, because the --version parameter is always required (I think), and for me --XX type parameters usually imply optionality. Not sure what the (drush) conventions are on parameters, but the following pattern might be more intuitive for users: drush fserver-distro [distro-name] [version-tag]