Document that read(2) can also return EPERM

See e.g. nfsclient/nfs_vnops.c

static int
nfs_read(struct vop_read_args *ap)
{
        struct vnode *vp = ap->a_vp;

        if (vp->v_type != VREG)
                return (EPERM);
        return (nfs_bioread(vp, ap->a_uio, ap->a_ioflag, ap->a_cred));
}

Approved by:	schweikh (mentor)
MFC after:	6 weeks

lib/libc/sys/read.2

@@ -163,6 +163,9 @@ was negative.
 .It Bq Er EAGAIN
 The file was marked for non-blocking I/O,
 and no data were ready to be read.
+.It Bq Er EPERM
+The file descriptor is associated with a non regular file
+(e.g. a directory) mounted over NFS.
 .El
 .Pp
 In addition,