docs(single): Updated docs for single with description and example (R…

…eactiveX#4616)

Updated docs for single with description and example

src/internal/operators/single.ts

@@ -11,8 +11,34 @@ import { Observer, MonoTypeOperatorFunction, TeardownLogic } from '../types';
  * items, notify of an IllegalArgumentException or NoSuchElementException respectively. If the source Observable
  * emits items but none match the specified predicate then `undefined` is emitted.
  *
+ * <span class="informal">Like {@link first}, but emit with error notification if there is more than one value.</span>
  * ![](single.png)
  *
+ * ## Example
+ * emits 'error'
+ * ```javascript
+ * import { range } from 'rxjs';
+ * import { single } from 'rxjs/operators';
+ *
+ * const numbers = range(1,5).pipe(single());
+ * numbers.subscribe(x => console.log('never get called'), e => console.log('error'));
+ * // result
+ * // 'error'
+ * ```
+ *
+ * emits 'undefined'
+ * ```javascript
+ * import { range } from 'rxjs';
+ * import { single } from 'rxjs/operators';
+ *
+ * const numbers = range(1,5).pipe(single(x => x === 10));
+ * numbers.subscribe(x => console.log(x));
+ * // result
+ * // 'undefined'
+ * ```
+ *
+ * @see {@link first}
+ *
  * @throws {EmptyError} Delivers an EmptyError to the Observer's `error`
  * callback if the Observable completes before any `next` notification was sent.
  * @param {Function} predicate - A predicate function to evaluate items emitted by the source Observable.