Improve ts.d documentation of Decomposition/Augmentation

[gwicksted]

Problem: it is complex to write a definition file for certain (very simplistic) js libs.

It is a weak point in the type definitions that, if not addressed by upcoming language features, should be more accessible in docs.

The concept of Decomposition/Augmentation needs more visibility in at least one of the Documentation pages since it's quite abstract.

Google usually links to the Modules docs when looking up info related to exports or ambient modules. Those docs are not that useful for authoring type definitions so it would be nice to link to the deep dive.

In #19372 @DanielRosenwasser offered a really good link that I don't think is part of the official docs (it should be a link at the very least!)

Unfortunately the bottom of the link mentioned above returns a 404

Example of a missing definition: gulp is quite popular and uses pumpify npm which uses this style of js. Yet pumpify does not have a DefinitelyTyped entry. Why not? Sure, it's not used that often but it likely has a lot to do with the difficulty of authoring the definition.

[RyanCavanaugh]

This doesn't seem actionable, nor a common complaint -- DT has 14,000+ contributors and nearly 7,000 types, so it can't be that hard to find a definition file for a library similar to the one being typed and crib from it.