field level comments are not present in jsdoc

If I remember correctly when `pbjs` generates JS code from protos and then `jsdoc` generates an HTML documentation, the comments for the individual fields in messages don't get to the resulting JS and HTML.

I tried to approach this a few years ago with https://github.com/googleapis/gapic-generator-typescript/pull/150 so we could probably resurrect that, or invent a different way of solving it :)