Update the man page to reflect the addition of NFSv4.2 (r355677).

Update all the references to NFSv4.1, so that they apply to NFSv4.1 and
NFSv4.2. Also, change the MDS->DS mounts to use NFSv4.2, so that both
versions of the protocol can be used against the server with pNFS enabled.

This is a content change.