Improve the documentation for the assertions

[sunesimonsen]

There seems to be some confusion around the sematics of was called with so we need to document that a bit better.

Mention that it uses to equal semantics by default and to satisfy sematics for matching arguments with expect.it(...) and link to the to satisfy assertion.

I also think we should recommend using to have calls satisfying instead in the top of the documentation.

[joelmukuthu]

Sorry for the confusion with the assignments here, I was working on something similar in #16 and thought it would cover this issue as well. Perhaps #25 covers it partially?