ueventd.rc: Document the different rule formats for /dev and /sys nodes

This was causing some confusion during shark bringup and we weren't able
to find docs online, so let's add some hints at the top of the file in
case it comes up again.

Change-Id: Ica2cd8a0fb28efb99077fdc98673dbbdd6f58ff6
Signed-off-by: Kevin Cernekee <cernekee@google.com>