Define how should look like the MT manual code.

In the new documentation @ckarlof defined the code to be:

    e3c5b

I think it should be 6 digits lengths because we can then display a digit keyboard to the phone user that will be easier for him to use also 6 digits lets us 10M possibilities which are more than enough if we only accept 2 or 3 tries before sending a new one.

Can we agree on changing that and make it 6 digits length for the manual code method?