Update vcdbg help
Update vcdbg help

The help text is now clearer.
When using vcdbg help <command>, prevent the command name itself
being displayed twice.

Fixes https://github.com/raspberrypi/firmware/issues/594

raspberrypi/userland,6by9/userland,raspberrypi/userland,6by9/userland,raspberrypi/userland,6by9/userland