Add should() second argument to docs

[oak-wildwood]

Document should() second argument

[ ] Bug report
[x] Content update
[ ] Process update (build, deployment, ... )

Adding should() second argument (from Mocha/Chai) to docs

We should add to our docs the lesser known second argument of should() assertion whereby you can pass in a failure message that will be picked up by Cypress. Should show an example of this in action.

Consider putting this as a snippet anywhere expect is discussed.