newgrp.1: Provide examples

"These examples appear to be originally noted thirteen years ago by
Mark Saad, contributed by eadler, put into a patch by Felix Johnson,
and made into mdoc by myself. Finally, put this bug to rest." ~ziaee

PR: 167742
Co-authored-by: eadler, ziaee, Mark Saad
Differential Revision: https://reviews.freebsd.org/D48877

(cherry picked from commit 86dc5dd0b2a861cf9d8cad18fc17c17dbbfc4465)