Documentation: nfs: fault_injection: convert to ReST

Convert fault_injection.txt to ReST and move it to admin-guide.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/f7b0cf8fb1159a668f75ce82a581e7590568c2b8.1578697871.git.dwlsalmeida@gmail.com


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