Expand MockTransport documentation [skip ci]

[machour]

I needed to test my API consumer, and found little to no documentation for MockTransport, so here's a first stab at it.

Would love some feedback if my overall code could be enhanced (is this the correct way to implement a mockable API client?)

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues

[machour]

I need to document $mock->flushRequests() too