docs(retry): add example for retry operator (#4691)

src/internal/operators/retry.ts

@@ -15,6 +15,36 @@ import { MonoTypeOperatorFunction, TeardownLogic } from '../types';
  * during failed subscriptions. For example, if an Observable fails at first but emits [1, 2] then succeeds the second
  * time and emits: [1, 2, 3, 4, 5] then the complete stream of emissions and notifications
  * would be: [1, 2, 1, 2, 3, 4, 5, `complete`].
+ *
+ * ## Example
+ * ```ts
+ * import { interval, of, throwError } from 'rxjs';
+ * import { mergeMap, retry } from 'rxjs/operators';
+ *
+ * const source = interval(1000);
+ * const example = source.pipe(
+ *   mergeMap(val => {
+ *     if(val > 5){
+ *       return throwError('Error!');
+ *     }
+ *     return of(val);
+ *   }),
+ *   //retry 2 times on error
+ *   retry(2)
+ * );
+ *
+ * const subscribe = example.subscribe({
+ *   next: val => console.log(val),
+ *   error: val => console.log(`${val}: Retried 2 times then quit!`)
+ * });
+ *
+ * // Output:
+ * // 0..1..2..3..4..5..
+ * // 0..1..2..3..4..5..
+ * // 0..1..2..3..4..5..
+ * // "Error!: Retried 2 times then quit!"
+ * ```
+ *
  * @param {number} count - Number of retry attempts before failing.
  * @return {Observable} The source Observable modified with the retry logic.
  * @method retry