named_attribute.7: Add a section for ZFS "sa" attributes

On ZFS, extended attributes with small values
(as in number of bytes) that are created when
xattr is set to "sa" will only be visible via extattr(2)
and not as named attributes.

This man page patch documents the above.

This is a content change.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D51444