named_attribute.7: Add an explanation for a quirk

[rmacklem]

Summary

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.

[kib]

share/man/man7/named_attribute.7

156

Perhaps start a new paragraph there, or even make this a new bullet point (.It).

[rmacklem]

Make separate bullet points as suggested by kib@