pjsip: Clarify certificate configuration for Websocket.

The Websocket transport uses the built-in HTTP server. As a result
the TLS configuration is done in http.conf and not in pjsip.conf.

This change adds a warning if this is configured in pjsip.conf and
also clarifies in the sample configuration file.

Change-Id: I187d994d328c3ed274b6754fd4c2a4955bdc6dd9