refactor(Assertion): small edit and improve docs
refactor(Assertion): small edit and improve docs

- Rename third parameter of Assertion constructor from `stack` to
  `ssfi` for consistency's sake.
- Add documentation to Assertion constructor explaining what the `object`,
  `message`, and `ssfi` flags are for.

meeber/chai,meeber/chai,lucasfcosta/chai,lucasfcosta/chai,chaijs/chai,chaijs/chai