named_attribute.7: Add an explanation for a quirk

I thought I would be able to come up with a
ZFS patch to avoid the possibility of there
being two attributes with the same name for
a file. This can only occur if the two attributes
are created when xattr=sa and xattr=dir respectively.

I have not been able to fix this, so this patch
documents this quirk.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D51751