Clarify the documentation RE: SWMR and NFS

[gheber]

For example, the HDF5 SWMR User's Guide contains pronouncements such as "The SWMR feature does not
work on network file systems (NFS, SMB/Windows file shares, and so on) since write ordering may not
be preserved on those systems." This sounds like a pronouncement, but it is vague and potentially misleading to users since, for example, there are NFS appliances that support SWMR.

[bikashrc25]

@gheber Any update on the SWMR documentation to include NFS with Pure can support SWMR? Thanks