Add a small section before the ZFS quota examples to mention that the users must exist to make the examples work without errors

[bcr]

Summary

PR177514 points out that the examples to set a quota for a home directory need to have the users already existing. Add a small paragraph before the examples to explain that.

Test Plan

1. Apply patch
2. build handbook
3. check output

I ran textproc/igor over the added section.
I'll commit this in 5 days if there a no further comments as this PR is already quite old.