message: docs(retryWhen): Add description
  Added a simple description about for what the operator is useful.
diff: @@ -18,6 +18,8 @@ import { MonoTypeOperatorFunction, TeardownLogic } from '../types';
  *
  * ![](retryWhen.png)
  *
  + * Retry an observable sequence on error based on custom criteria.
  + *
  * ## Example
  * ```ts
  * import { timer, interval } from 'rxjs';
  