Add docstring to EventStore events

Problem to solve

• Our event schemas are a form of public interface
• The name of most events will be sufficient to describe when the event is raised (e.g. MergeRequest::ApprovedEvent is obviously raised when a merge request is approved)
• However some events will need an additional description to describe how they're triggered

Proposal

• Add a doc string to each EventStore event
• Capture this in a way such that it's programmatically accessible (i.e. not buried in a # comment)