docs(onErrorResumeNext): create documentation for the operator (#2571)

Create documentation for 'onErrorResumeNext' operator. Describe
what it accepts and returns. Compare it to 'concat' operator. Add
note about 'catch' operator. Provide example of usage.

Author: mpodlasin

src/operator/onErrorResumeNext.ts

@@ -16,6 +16,69 @@ export function onErrorResumeNext<T, T2, T3, T4, T5, T6, R>(this: Observable<T>,
 export function onErrorResumeNext<T, R>(this: Observable<T>, ...observables: Array<ObservableInput<any> | ((...values: Array<any>) => R)>): Observable<R>;
 export function onErrorResumeNext<T, R>(this: Observable<T>, array: ObservableInput<any>[]): Observable<R>;
 /* tslint:enable:max-line-length */
+
+/**
+ * When any of the provided Observable emits an complete or error notification, it immediately subscribes to the next one
+ * that was passed.
+ *
+ * <span class="informal">Execute series of Observables no matter what, even if it means swallowing errors.</span>
+ *
+ * <img src="./img/onErrorResumeNext.png" width="100%">
+ *
+ * `onErrorResumeNext` is an operator that accepts a series of Observables, provided either directly as
+ * arguments or as an array. If no single Observable is provided, returned Observable will simply behave the same
+ * as the source.
+ *
+ * `onErrorResumeNext` returns an Observable that starts by subscribing and re-emitting values from the source Observable.
+ * When its stream of values ends - no matter if Observable completed or emitted an error - `onErrorResumeNext`
+ * will subscribe to the first Observable that was passed as an argument to the method. It will start re-emitting
+ * its values as well and - again - when that stream ends, `onErrorResumeNext` will proceed to subscribing yet another
+ * Observable in provided series, no matter if previous Observable completed or ended with an error. This will
+ * be happening until there is no more Observables left in the series, at which point returned Observable will
+ * complete - even if the last subscribed stream ended with an error.
+ *
+ * `onErrorResumeNext` can be therefore though of as version of {@link concat} operator, which is more permissive
+ * when it comes to the errors emitted by its input Observables. While `concat` subscribes to the next Observable
+ * in series only if previous one successfully completed, `onErrorResumeNext` subscribes even if it ended with
+ * an error.
+ *
+ * Note that you do not get any access to errors emitted by the Observables. In particular do not
+ * expect these errors to appear in error callback passed to {@link subscribe}. If you want to take
+ * specific actions based on what error was emitted by an Observable, you should try out {@link catch} instead.
+ *
+ *
+ * @example <caption>Subscribe to the next Observable after map fails</caption>
+ * Rx.Observable.of(1, 2, 3, 0)
+ *   .map(x => {
+ *       if (x === 0) { throw Error(); }
+         return 10 / x;
+ *   })
+ *   .onErrorResumeNext(Rx.Observable.of(1, 2, 3))
+ *   .subscribe(
+ *     val => console.log(val),
+ *     err => console.log(err),          // Will never be called.
+ *     () => console.log('that\'s it!')
+ *   );
+ *
+ * // Logs:
+ * // 10
+ * // 5
+ * // 3.3333333333333335
+ * // 1
+ * // 2
+ * // 3
+ * // "that's it!"
+ *
+ * @see {@link concat}
+ * @see {@link catch}
+ *
+ * @param {...ObservableInput} observables Observables passed either directly or as an array.
+ * @return {Observable} An Observable that emits values from source Observable, but - if it errors - subscribes
+ * to the next passed Observable and so on, until it completes or runs out of Observables.
+ * @method onErrorResumeNext
+ * @owner Observable
+ */
+
 export function onErrorResumeNext<T, R>(this: Observable<T>, ...nextSources: Array<ObservableInput<any> |
                                                        Array<ObservableInput<any>> |
                                                        ((...values: Array<any>) => R)>): Observable<R> {