[Documentation request] Interactions with fakeAsync and HttpClientTestingModule

[GeeWee]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[ ] Feature request
[X] Documentation issue or request
[ ] Support request
[ ] Other... Please describe:

Current behavior

Currently it's not quite easy to figure out how Cashew will behave in tests. What it's interactions are with HttpClientTestingModule, and verifications on this. It is also not clear how it will behave in regards to fakeAsync, tick and their interactions with TTL.

Expected behavior

Some documentation about these interactions would be most helpful in testing code that relies on Cashew.

What is the motivation / use case for changing the behavior?

When testing code using cashew and using HttpClientTestingModule it can be hard to know exactly how to use the primitives such as flush, and how to properly do assertions on the HttpTestingController