Document the 'type' query parameter for the "List webinars" API

The “List webinars” API currently does not document the type query parameter, but the API responds to its presence and behaves similarly to the equivalent parameter in the “List meetings” API. Please document this query parameter for webinars.

We are taking a dependency on this query parameter for webinars to retrieve all occurrences of upcoming webinars because omitting the type query parameter suppresses some of the occurrences.