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

Include references to NFSv4.2 and Flexible File layout, plus clarify
when vfs.nfsd.flexlinuxhack needs to be set for Linux pNFS clients.
Also update the man page to reflect the addition of SpaceUsed to the
attributes stored in the extended attribute on the MDS (r354158).

This is a content change.