Add explanation to the ZFS quota examples about /home.

The ZFS quota examples started with setting quotas for a user that was not
created in the chapter. This led to some confusion for (new) people who tried
out the example and got an error message about the missing user.
Add a small paragraph to explain this concept and that ideally, when creating
a user, the /home dataset should be created first to not shadow it afterwards.

PR: 177514
Submitted by: Bryan Bosworth
Reviewed by: noone (5 day review timeout)
Differential Revision: https://reviews.freebsd.org/D24913