doc: Explain behaviour of when/expect

[timabell]

I found the behaviour of when/expect confusing, and couldn't see how to set the "always" behaviour. This patch adds a bit more clarity to that area of usage to the readme.

Hopefully I've the updated description of the behaviour is accurate, I've not used this library much.

Thanks for the library, solves a painful problem.

[richardszalay]

Apologies for the delayed reply on this, but isn't this effectively duplicating the "Match Behavior" section of the readme?

[timabell]

Well, technically yes, but let me put it this way, I think I spent about 2 or 3 hours scratching my head why I wasn't getting a mock to hit when it was defined even after reading the existing docs. This proposed addition is how I would have liked it explained to me to be able get the mental model I needed to understand why the sequence of mocks behave as they do with the combinations of when / expect and the configuration.