Fix a misleading documentation of Observable.singleElement() (#5668)

* Fix a misleading documentation of Observable.singleElement()

* Reduce the first part of Observable.singleElement() JavaDoc to a single sentence.

* Update the Marble diagram of Observable.singleElement()

Author: VeskoI

src/main/java/io/reactivex/Observable.java

@@ -10277,11 +10277,10 @@ public final Observable<T> share() {
     }
 
     /**
-     * Returns a Maybe that emits the single item emitted by this Observable if this Observable
-     * emits only a single item, otherwise if this Observable emits more than one item or no items, an
-     * {@code IllegalArgumentException} or {@code NoSuchElementException} is signalled respectively.
+     * Returns a Maybe that completes if this Observable is empty or emits the single item emitted by this Observable,
+     * or signals an {@code IllegalArgumentException} if this Observable emits more than one item.
      * <p>
-     * <img width="640" height="315" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/singleElement.png" alt="">
+     * <img width="640" height="217" src="https://raw.githubusercontent.com/wiki/ReactiveX/RxJava/images/rx-operators/singleElement.o.png" alt="">
      * <dl>
      *  <dt><b>Scheduler:</b></dt>
      *  <dd>{@code singleElement} does not operate by default on a particular {@link Scheduler}.</dd>

src/test/java/io/reactivex/internal/operators/observable/ObservableSingleTest.java

@@ -282,6 +282,7 @@ public void testSingleWithEmpty() {
 
         InOrder inOrder = inOrder(observer);
         inOrder.verify(observer).onComplete();
+        inOrder.verify(observer, never()).onError(any(Throwable.class));
         inOrder.verifyNoMoreInteractions();
     }