Mention numeric password condition for PlaMo and Phosh

Common question in #main:postmarketos.org is "What's the default password in Phosh/PlaMo?" and when answered with "It's your user password", common reply was "But lockscreen keyboard doesn't have letters!". This very understandable, people didn't have any way of knowing that only numeric passwords are supported in those UIs, but now people should be able to see that during pmbootstrap init, so this kind of questions should be less common