Clarify the documentation RE: SWMR and NFS

For example, the [HDF5 SWMR User's Guide](https://support.hdfgroup.org/releases/hdf5/documentation/features/SWMR/HDF5_SWMR_Users_Guide.pdf) 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.