Remove part of docstring referring to removed funcitonality

We no longer have requirement of at least two groups in Group Configuration, since https://github.com/edx/edx-platform/pull/4920

One unchanged test slipped, and had to be corrected as a part of https://github.com/edx/edx-platform/pull/4948/files#diff-4dd7256fb6aa09766356f8a475159589L648

This PR fixes related docstring.

@olmar @auraz