Consider documenting `g::c::Options` in a dedicated doc

[dbolduc]

Options, as a class, are pretty well documented.

We might want to make a new doc (e.g. doc/options.dox) to expand on it though. We can go into more details on the different types of Options (per-call options, client options, connection options) and how they are applied. We can also highlight commonly used Options.

Maybe a helpful example use case could be: "How to share a background thread pool across multiple clients". (by setting the GrpcCompletionQueueOption and passing it into Make*Connection(options))

[coryan]

Keep it open. Maybe for a doc fixit.

[coryan]

I think https://googleapis.dev/cpp/google-cloud-common/HEAD/group__options.html fixes this problem. Closing.