Truncated requests may only be retried if route is idempotent.

* Update retry logic documentation to reflect this.

* Previously, truncated requests would always be retried. This could
  have caused the requested action to be repeated twice. However,
  perhaps this change is somewhat academic since I don't know if short
  responses can or are likely to get truncated in arbitrary places, and
  most or all requests with long responses are idempotent anyway.