[docs] Specify the "service" used for PAM

[tmccombs]

While the polkit(8) manpage mentions that polkit uses pam, it doesn't provide much detail on how pam can be configured for polkit.

After some digging, I believe that it uses the "polkit-1" service name, and thus can be configured with /etc/pam.d/polkit-1.

I think that the man page should probably mention that, so that users can know where to change pam settings specifically for polkit.

For example, to allow polkit authorization to allow just using a yubikey or fingerprint scanner, even if that isn't sufficient for normal login.