Update outdated example in HTTP API documentation

[oxzi]

The changes to Event.Validate in Icinga/icinga-go-library#212 - introduced here via #442 - invalidated the curl example in the HTTP API documentation. The example payload now errors out with "invalid event: at least one of 'incident' or 'muted' must be set".

[yhabteab]

The event type should also be fully documented, like listing all available fields with their corresponding descriptions.