Documentation: nfs: nfs41-server: convert to ReST

Convert nfs41-server.txt to ReST. ASCII tables were converted to ReST grid
table format.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200129044917.566906-5-dwlsalmeida@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>