update docs to show that ES modules can be used with root hook plugins

[boneskull]

~~With #4244 landing and #4304 thereafter, we should ensure that root hook plugins can be used with ES modules; e.g. export const mochaHooks = {..}

I'll take care of this. Should probably land for v8, as it would be confusing if it didn't. ~~

It does work, and there are even tests for it.

What's missing is docs. I'll get to that.

[boneskull]

Actually I think it does work. wow, good job

[boneskull]

Hmm, I don't like the failure mode when --require some-esm.js (instead of .mjs). The error is unrelated and unhelpful

[boneskull]

closed via #4311