Consistent inline code formatting in js-sys docs (#1915)

* Consistent inline code formatting in docs

Also corrects the docstring of `Math.cbrt()` (the cubic root of x is not x^3).

* Add backticks to Array#includes()

* Fix links to functions in Reflect docs

As soon as intra-rustdoc links land in stable the round brackets can be removed.

Author: haveyaseen