virtiofsd: document cache=auto default

The virtiofsd --help output documents the cache=auto default value but
the man page does not. Fix this.

Signed-off-by: Harry G. Coin <hgcoin@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200916112250.760245-1-stefanha@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

docs/tools/virtiofsd.rst

@@ -103,6 +103,7 @@ Options
   forbids the FUSE client from caching to achieve best coherency at the cost of
   performance.  ``auto`` acts similar to NFS with a 1 second metadata cache
   timeout.  ``always`` sets a long cache lifetime at the expense of coherency.
+  The default is ``auto``.
 
 Examples
 --------