GitLab Webhooks documentation

The above link shows the documentation on the output of gitlab webhooks, it is fair and clean.
But, it doesn’t specify the received parameters in details, like for eg. when a Issue Hook is sent, what are the possible combinations for a ‘action’ attribute as such.

This gets increasingly harder to frame a high level api to respond to web hooks. Consider the GitHub documents,
they are much cleaner and are well understood upon the first look.

Please help us with good documentation.