Update the content of the documentation to clarify how the cache option
from virtiofsd has an impact on the host page cache, while the dax
option for the filesystem in the guest has an impact on the guest page
cache.
The document is also updated to not have lines exceeding 80 characters.
Fixes#3826
Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>