Correct the description of characters allowed.  Based on pw_checkname in
usr.sbin/pw/pw_user.c.  Modified version of patch submitted by
venture37.

PR:		docs/47594
Submitted by:	Fernando Schapachnik <fernando@mecon.gov.ar>, venture37 <venture37@geeklan.co.uk>
Reviewed by:	allanjude, bcr, brueffer (on phabricator)
MFC after:	1 week