Document how hyper is using UpgradedSendStream safely

[erickt]

Is your feature request related to a problem? Please describe.

On Fuchsia, we are updating our vendored version of Hyper from 0.13 to 0.14 in https://fuchsia-review.googlesource.com/c/fuchsia/+/611683/. As part of this we are reviewing the hyper change. One portion of the codebase we are struggling with is src/proto/h2. In #2523, @nox introduced the unsafe UpgradedSendStream and Neutered, and it's not straightforward what it does, and how we know hyper is using it correctly.

Describe the solution you'd like

There's a nice discussion though in #2523 that gives some context. It would be helpful if a version of this discussion was added to the code. That would make it easier to review vendored code.