Add user experience considerations

[PieterKas]

First item to add is guidance on limited input devices.

[PieterKas]

@gffletch I recall we discussed this, but unable to recall the details of the concern on what guidance we had in mind.

[aaronpk]

Notes from editor's call:

• "Not recommended to use this with challenges that require significant user experience challenges, for example entering a password on a TV screen with an on-screen keyboard."
• Security consideration around the differences between exposure of credentials on a TV screen in a room vs the risk of a cross-device flow
• We don't want to discourage the possibility of using this flow with a passkey on a TV remote control
• "If you're on a limited input device, and the authentication method the user must pass requires significant user interaction, e.g. entering a password, then you shouldn't use this method; use the device flow instead."

[aaronpk]

Completed in #60