I'm sure someone could do better than this but here is a first try at a suitably informative prompt:
$site_name wants to use WebRTC.
WebRTC allows voice calling, video chat, and P2P file
sharing, but can also be a privacy risk. We recommend
allowing WebRTC only on sites that you expect to use
such features on.
[Link to learn more]
Allow WebRTC for $site_name?
Something like Flash's audio/video access prompt would also be a good idea:
https://voicethread.com/image/howto/flash_settings_camera_mi...