MFC r343921:
Add an example to pw.8 about how to add an existing user to a group.

Instead of using pw to modify group membership, users often edit
/etc/group by hand, which is discouraged. Provide an example of
adding a user to the wheel group, which is a common use case.
I'm using a different user here as in the previous example as that
deleted the user (although the examples don't necessarily have to
be followed in order).

Reviewed by: rgrimes,0mp
Approved by: 0mp
Differential Revision: https://reviews.freebsd.org/D19123