FAQ: YubiKey 4 does not support asking for pin
FAQ: YubiKey 4 does not support asking for pin

Explicitly state YubiKey 4 series does not support asking for pin.

__Problem__
* FIDO(1) devices are only partially supported. They can be used as a second authentication factor to the password or as a single factor by themselves, but they cannot be used in conjunction with a `pin`.

* The failure mode consists in silently ignoring the device.
It does not provide any feedback as to why the authentication procedure is failing and it's thus very complicated to debug (see for example #144).

* YubiKey product pages, understandably, do not highlight the differences between FIDO and FIDO2, users may thus not realize at a first glance what their devices do actually support.

__This PR__
Add a paragraph to help users like myself understand why their YubiKeys work only half of the time.

__Next steps__
Modify `pamu2fcfg` to emit a warning when registering devices with unsupported configurations.
Yubico/pam-u2f,Yubico/pam-u2f,Yubico/pam-u2f