Document that a TestStep is either a PickleTestStep or a HookTestStep

[mpkorstanje]

A TestStep is either a PickleTestStep or a HookTestStep.

• When a PickleTestStep then the id, pickleStepId, stepDefinitionIds and stepMatchArgumentsLists are required.
• When a HookTestStep then the id and hookId are required.

So in the union of these, only the id is required.

There are now better ways to express this in a json schemas but either we didn't have those at the time, or they didn't quite work.

Originally posted by @mpkorstanje in #249 (comment)