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>

rootdir/ueventd.rc

@@ -1,6 +1,13 @@
 subsystem adf
 	devname uevent_devname
 
+# ueventd can only set permissions on device nodes and their associated
+# sysfs attributes, not on arbitrary paths.
+#
+# format for /dev rules: devname mode uid gid
+# format for /sys rules: nodename attr mode uid gid
+# shortcut: "mtd@NN" expands to "/dev/mtd/mtdNN"
+
 /dev/null                 0666   root       root
 /dev/zero                 0666   root       root
 /dev/full                 0666   root       root