Docs: update js.md

Added some more information about plugins.js to the docs
> …to an noop…" should be "…to a noop…"

@QWp6t I [committed this with some modification](https://github.com/h5bp/html5-boilerplate/commit/5027eeaab2c1873de8ff48d5c87411be6f26779a), and that part didn't get into the docs. :)