Document cibuild and release scripts, make them testable

[shawnbot]

The cibuild and release scripts (release, release-candidate, and release-pr) need some more descriptive comments of what's happening in them so that we can reason about them when reviewing changes. It'd also be nice to either have docs for how to test them locally or actually make them testable locally, i.e. in some way that they don't actually publish to npm when they're not running on Travis unless you explicitly tell them to. (Alternatively, there could be some sort of --dry-run-like option that they could recognize, which would skip or otherwise disable the publish step.)

This might also be a good place to describe the release logic that happens on Travis in a more human-readable way. 🤓 🤖

[shawnbot]

These scripts are dramatically simplified as of #625, so there shouldn't be any need to document them or make them testable on their own.