The documentation should explain which tests are timed, triggered by upload (or merge), which are triggered by Gerrit comments, and how to trigger any job manually on Sandbox or Production Jenkins.
Explain what does the job output looks like. Which parts of console output to investigate, what artifacts are archived and what to look for in them.
Which jobs are useful for VPP developers, CSIT contributors, and which (if any) are "for experts only".

For triggering, explain any special syntax, required fields, and pitfalls (e.g. to avoid triggering too many tests at once).
Explain quirks of Gerrit-Jenkins interaction. Maybe explain physical testbed reservation.

The end result can be implemented as a set of documents linking to each other, maybe with one "where do I start" document.