statfs.2: Update man page for the MNT_NAMEDATTR flag

This patch updates the man page for the MNT_NAMEDATTR flag.

Another man page that explains named attributes will
be introduced in a future commit.

This is a content change.

Reviewed by: manpages (zaiee)
Fixes: 2ec2ba7e232d ("vfs: Add VFS/syscall support for Solaris style extended attributes")
Differential Revision: https://reviews.freebsd.org/D49719