Add documentation

[cachapa]

Add documentation to most, or all public methods and fields using the /// comment notation, e.g.:

/// Add [x] to [y] and return the result
int add(int x, int y) {
  return x + y;
}

It would be nice to document at least 20% of the public API to satisfy pub.dev score.

The challenge is to find a way for pub.dev to ignore the generated code, or to change the way code generation works so it's no longer part of the Firedart SDK.

Ideally, doing the above will also organize the pub.dev API documentation which currently is essentially unusable.

[nickmeinhold]

When I run pana locally I get:

702 out of 2214 API elements (31.7 %) have documentation comments.

and 160/160 points.

I'd be happy to add some basic documentation to the public API if you still want this but thought I would check first given pana is happy.

[cachapa]

I would appreciate that. As described above, one of the issues is that the generated code represents a large part of this project and I'm not sure if there's a way to tell the tooling to ignore that (maybe it's already doing that?).

In any case documenting our public API is always welcome.