Documentation: nfs: knfsd-stats: convert to ReST

Convert knfsd-stats.txt to ReST. Content remains mostly the same.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200129044917.566906-6-dwlsalmeida@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>