Document that `const c2 = vm.createContext(c1); c1 === c2` is true

As far as I can tell, the documentation didn't say this, so I was left wondering if there is a difference between c1 and c2 in const c2 = vm.createContext(c1); According to my own tests, they are equivalent. I thought it was worth documenting as it would've saved me a bit of time if this information were written down.