Improve documentation for jsonnet service definition

The presence of default fields and values tells only half the story. Having a schema and its possible values would help immensely to figure out how to fill them out (perhaps using JSON Schema, as we already use it to validate some yaml files).

What and why each field is present might be useful to declare for some fields where the purpose is less obvious, as part of the information present at the moment relies on tacit knowledge. It may be obvious for some engineers and less obvious for the rest.

Link(s) to possible places where to find the information we are looking at that needs to be filled is also relevant, although difficult (not to say impossible) to enforce as it is usually free form and can be composed from multiple sources.