Upgrade documentation for unlocking wallet

[Desrever Nu]

and follow masterofdisaster suggestion

Unlocking via commandline (e.g. on a headless RaspberryPi):
The two Nu wallets are locked/unlocked separately. To unlock the NuBits wallets, make sure to execute the command in the NuBits context; type:

nud -unit=B walletpassphrase <your passphrase> 9999999999 true