Improve existing documentation and add copyeditor

[davidtheclark]

• Improves some punctuation and phrasing in the existing shortcut documentation.
• Adds @mapbox/copyeditor to help with that: fixes all the warnings it gives now and adds a documentation-building-and-copyediting step to the pretest script.
• Changes ambiguous links to headings like #parameters so they point to the shortcut's top-level heading. Because that heading is the shortcut's name, it will not be duplicative, so we won't break links as we add more shortcuts. This also makes the link more predictable, which will be helpful if we try to transform those link destinations when putting these docs in order contexts.

@rclark for review, please.

[davidtheclark]

I noticed that the options object param was being labeled as optional (not required) because they had default parameters: (options = {}). In every case, the options object is actually required, so I removed the default parameter in https://github.com/mapbox/cloudfriend/pull/97/commits/a534c12a83f27709c8cadf12198b75caf25e5f77.

... And this broke tests. I'll think a little.

[davidtheclark]

The conventional input validation approach in these functions takes that default parameter for granted. And it looks like the hope in documentation.js is that you can use the standard JSDoc @name tag — which we could do, but that turns off all inference so we'd also have to manually add @augments tags, instead of letting documentation.js pick up on the extends keyword. So, a few options:

1. Keep the default parameter, add @name to every JSDoc comment and @augments to a bunch.
2. Remove the default parameter, add a new little convention to validate that options exists at all and throw a clear error if not (e.g. if (!options) throw new Error('Options are required'), and change tests to account for that new validation.
3. Same as (2), but instead of expanding the validation conventions here, switch over to a tool like fusspot.

I think that unless someone is enthusiastic about (3), I'm in favor of (2), and will give that a shot.