Update the documentation for the memory_stats call.
Update the documentation for the memory_stats call.

Also add a FIXME comment on a more rubyish way we could
do things.

Signed-off-by: Chris Lalancette <60b62644009db6b194cc0445b64e9b27bb26433a@redhat.com>

pronix/ruby-libvirt,pronix/ruby-libvirt,pronix/ruby-libvirt