pw: Clarify groupadd/groupmod memberlist formats

Clarify in which formats the memberlist for groupadd/groupmod can be
accepted for '-M', '-m', or '-d' flag.

Related commit: 40ab104eaa6f6d5577dcff6d8bf04de03faef345.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1352