`declare global` is not explained anywhere

[adriancuadrado]

Page URL: https://www.typescriptlang.org/docs/handbook/declaration-files/by-example.html

Issue: It doesn't explain what declare global is, how to use it, how it works, and when and why you should use it.

I think this is the most appropriate place to explain this, but I am not even sure about that. Maybe it might fit somewhere else, I don't really know.

This construct is mentioned here, but only as an example and doesn't really get into details about its mechanics.