[Encore] Document exposing global variables

[weaverryan]

In many cases, you may need to do this:

const $ = require('jquery');
window.$ = window.jQuery = $;

so that you can use $ or jQuery outside of a webpack file (e.g. because you have some JS in a template). We need to document this in the "legacy" section.

[stof]

I would recommend this instead:

const $ = require('jquery');
global.$ = global.jQuery = $;

This would let us add window.jQuery again in the autoProvideJquery list, to support bootstrap by default