Documentation: nfs: rpc-cache: convert to ReST

Convert rpc-cache.txt to ReST. Changes aim to improve presentation
but the content itself remains mostly the same.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200129044917.566906-3-dwlsalmeida@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>