To turn off the NFS attribute cache in FreeBSD, one has to set the
following four flags to zero in struct nfs_args:

nfs_args->acregmin = 0;
nfs_args->acregmax = 0;
nfs_args->acdirmin = 0;
nfs_args->acdirmax = 0;

In addition, you have to turn on four flags to tell the kernel that you
indeed want it to use your values for the above four flags:

nfs_args->flags |= \
  (NFSMNT_ACREGMIN|NFSMNT_ACREGMAX|NFSMNT_ACDIRMIN|NFSMNT_ACDIRMAX);

This is rather cumbersome and also confusing because some users may be
used to what other OSs do: just setting the four fields suffices.  For
that reason, some OSs like Solaris provide a convenience flag called
"noac" that can be set to simply turn off the attribute cache entirely.

I would like to ask that FreeBSD support such a convenience flag "noac".  

In addition, please document this flag and the current behaviour of how
one turns off the attribute cache, in the mount_nfs man page.