NFS: add comments clarifying the use of nfs_post_op_update()

Comments-only change to clarify a detail of the NFS protocol and how it is
implemented in Linux.

Test plan:
None.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>