open.2: Document the O_NAMEDATTR flag

Commit 2ec2ba7e232d added support for the O_NAMEDATTR flag to
the open(2) and openat(2) syscalls for main.

This patch updates the man page for this flag.

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

This is a content change.

Reviewed by: emaste (earlier version), kib (earlier version)
Differential Revision: https://reviews.freebsd.org/D49691
Fixes: 2ec2ba7e232d ("vfs: Add VFS/syscall support for Solaris style extended attributes")