install: Write more explicitly what the passwords are for ('user' and full disk encryption)

Created by: ollieparanoid

@PureTryOut has a good idea:

I finally realize what the second password is for: to decrypt the just newly made encrypted partition. lol. could that maybe be mentioned somehow? it's a bit confusing otherwise

We should also explicitly write, what the user password is for, before asking for it. Especially because we run sudo a lot, which also asks for a password.