feat(docs): Add some basic API documents.

@zaach, @6a68, @nchapman - could I get a review of these? I'm adding @6a68 and @nchapman because they need some docs NOW.
> "The promise will be redirected if a required parameter is missing."

? Should that say "The promise will be _resolved_ if a ..."? (same comment re: line 38 below)