Add documentation on which jstest tags have which passthrough effects

For engineers on Query it is a pain point for us to add new tests to jscore. With the increasing number of passthroughs and jstest tags, it's not easy to tell at-a-glance what tags are available and/or might be relevant for someone writing a new test.

This is a request to add some sort of documentation about the test tags. It would be cool for this to be done automatically somehow by parsing our YAML definitions, so that newly-added tags are automatically documented somewhere.