Are you using the Community Edition, or the Enterprise Edition of Activiti? The process will vary between the two - and potentially which version number you're using.
With Community Edition you will need to create a single task (typically a script task) process model and assign the target namespace to "activiti-report". There are examples in activiti-webapp-explorer2 under src/mair/resources/org/activiti/explorer/demo/process/reports. There is an example of this in one of Joram's Blogs from version 5.12 of Activiti.
With the Enterprise Edition, there are some examples in the Github repository, as either their own Java class, or just in test cases for more of the specific content you're asking about. There is also a section of the Developer Guide dedicated to what you're looking into.
I do not believe there are actual online API documents pertaining to the classes you're asking about, as they're a part of the Enterprise version of the product, and thus not open for free utilization.