JSDoc for the sw-broadcast-cache-update project.

[jeffposnick]

R: @addyosmani @gauntface

Here's an initial batch of JSDocs, for the sw-broadcast-cache-update project.

You can take a look at them rendered at https://github.com/GoogleChrome/sw-helpers/blob/jsdoc/projects/sw-broadcast-cache-update/README.md

It unfortunately feels like a pretty noisy page, and probably isn't the best format to share with developers, but that's just one possible rendering of the underlying JSDocs (via the documentation library). I think @gauntface has alternative plans for how they'll get rendered in the future.

The one sore point that I ran into was that documentation forces the name $0for the anonymous destructed object we're using as parameters in all the functions. That $0 could be confusing, but it's not currently possible to override. Maybe we should consider post-processing the generated docs to convert $0 to parameters or something?