Document the health checks API options

* `initialDelaySeconds` is currently named `gracePeriodSeconds`
* `acceptableReponses` is not exposed in the API; it is hard-coded to
  [200 - 399]

Related to #305.
Make “health” “healthy” and wrap at 80 chars
Merge pull request #341 from mesosphere/document-health-checks

Document the health checks API options