Improve `RetryAfter` docs

[mattkime]

Two bits -

I was trying to set RetryAfter and failed to figure out how. I don't think its hard but something in the docs must be confusing me. I'm particularly concerned with setting it for an octokit instance.

The design of the default retry interval is pretty damn smart and should be document. I wouldn't have mucked with RetryAfter if I had known. For three retries, its hard to discern the retry interval. With a larger number of retries it was easy to discern the n^2 pattern.