WebRtcEndpoint: document the 'pemCertificate' settings
WebRtcEndpoint: document the 'pemCertificate' settings

It is possible to configure an external certificate to be used by DTLS.
This is important because some browsers such as Firefox won't allow to
establish multiple connections from the same tab to different peers that
use different DTLS certificates.

Kurento/kms-elements,Kurento/kms-elements,Kurento/kms-elements