cros-gunzip-dd: add some doc

cros-gunzip-dd.py

@@ -4,7 +4,9 @@
   Usage: cros-gunzip-dd.py SRC.gz DEST
 
   It's a shortcut for `zcat SRC.gz | dd of=DEST`. Useful on nayuos because the
-  "dd" command may crash.
+  "dd" command may crash. Unlike "dd", this script does not writing
+  synchronously, you may want to wait for cache to be emptied after the copy.
+  You can still force your system to empty its caches by using `sync` command.
 """
 import sys, gzip
 src = gzip.open(sys.argv[1], "rb")