Document the adb install -d option.

Change-Id: Ife0da42c22e4408dc5c07ac0f20e0ecbbde20832
diff --git a/commandline.c b/commandline.c
index c9bb437..61167b2 100644
--- a/commandline.c
+++ b/commandline.c
@@ -133,10 +133,11 @@
         "  adb forward --remove <local> - remove a specific forward socket connection\n"
         "  adb forward --remove-all     - remove all forward socket connections\n"
         "  adb jdwp                     - list PIDs of processes hosting a JDWP transport\n"
-        "  adb install [-l] [-r] [-s] [--algo <algorithm name> --key <hex-encoded key> --iv <hex-encoded iv>] <file>\n"
+        "  adb install [-l] [-r] [-d] [-s] [--algo <algorithm name> --key <hex-encoded key> --iv <hex-encoded iv>] <file>\n"
         "                               - push this package file to the device and install it\n"
         "                                 ('-l' means forward-lock the app)\n"
         "                                 ('-r' means reinstall the app, keeping its data)\n"
+        "                                 ('-d' means allow version code downgrade)\n"
         "                                 ('-s' means install on SD card instead of internal storage)\n"
         "                                 ('--algo', '--key', and '--iv' mean the file is encrypted already)\n"
         "  adb uninstall [-k] <package> - remove this app package from the device\n"