text: Title: docs: explain idempotent requests
  Body: Idempotent requests are a standard of the REST POST and PUT api's wherein duplicate requests or re-tried requests for certain use cases have a single effect and outcome. For example, user account creation is unique and retries will return an error response. In a similar way asynchronous jobs that have the same identifier will only be scheduled to run once. This concept must be documented in jina so that the custom Executors are aware that retrying requests with side effects are carefully handled.