adduser(8): Add documentation for ZFS encrypted home dataset

[john.grafton_runbox.com]

Summary

Fill in missing documentation on how to configure a ZFS encrypted
home dataset to auto mount during login and provide an example
configuration through commented lines in PAM configuration that
match test plan from https://reviews.freebsd.org/D28018

PR: 282957
Reported by: Mike Watkins <[e-mail]>

[grahamperrin]

usr.sbin/adduser/adduser.8

148

150

151–152

153–154

163

166

167

168

169

usr.sbin/adduser/adduser.sh

260–261

261

639

[john.grafton_runbox.com]

Updated language to match Graham's suggestions

[grahamperrin]

Thanks. With all inlines hidden, the language looks OK, to me.

(I can't comment on the effects; I have not tested.)