You will find an introduction as well as a full reference in the latest cdist manual.
The community-maintained 'contrib' types are documented on their own manual.
Experimental features from the beta branch (i.e. cdist trigger, preos and scan) are documented under the beta manual.
Depending how you installed cdist, type manuals might also be availale via the
man 7 cdist-type__file).
Manual for previous releases can be found on the manual archives page.
You will find the full changelog here.