2.x: Fix toFlowable marbles and descriptions (#6200)

* 2.x: Fix toFlowable marbles and descriptions

* Adjust wording

Author: akarnokd

Diff for: src/main/java/io/reactivex/Flowable.java

@@ -5842,15 +5842,16 @@ public final T blockingSingle(T defaultItem) {
     }
 
     /**
-     * Returns a {@link Future} representing the single value emitted by this {@code Flowable}.
+     * Returns a {@link Future} representing the only value emitted by this {@code Flowable}.
+     * <p>
+     * <img width="640" height="324" src="https://github.com/ReactiveX/RxJava/wiki/images/rx-operators/Flowable.toFuture.png" alt="">
      * <p>
      * If the {@link Flowable} emits more than one item, {@link java.util.concurrent.Future} will receive an
-     * {@link java.lang.IllegalArgumentException}. If the {@link Flowable} is empty, {@link java.util.concurrent.Future}
-     * will receive a {@link java.util.NoSuchElementException}.
+     * {@link java.lang.IndexOutOfBoundsException}. If the {@link Flowable} is empty, {@link java.util.concurrent.Future}
+     * will receive a {@link java.util.NoSuchElementException}. The {@code Flowable} source has to terminate in order
+     * for the returned {@code Future} to terminate as well.
      * <p>
      * If the {@code Flowable} may emit more than one item, use {@code Flowable.toList().toFuture()}.
-     * <p>
-     * <img width="640" height="395" src="https://github.com/ReactiveX/RxJava/wiki/images/rx-operators/B.toFuture.png" alt="">
      * <dl>
      *  <dt><b>Backpressure:</b></dt>
      *  <dd>The operator consumes the source {@code Flowable} in an unbounded manner

Diff for: src/main/java/io/reactivex/Observable.java

@@ -5340,15 +5340,16 @@ public final T blockingSingle(T defaultItem) {
     }
 
     /**
-     * Returns a {@link Future} representing the single value emitted by this {@code Observable}.
+     * Returns a {@link Future} representing the only value emitted by this {@code Observable}.
+     * <p>
+     * <img width="640" height="312" src="https://github.com/ReactiveX/RxJava/wiki/images/rx-operators/toFuture.o.png" alt="">
      * <p>
      * If the {@link Observable} emits more than one item, {@link java.util.concurrent.Future} will receive an
-     * {@link java.lang.IllegalArgumentException}. If the {@link Observable} is empty, {@link java.util.concurrent.Future}
-     * will receive an {@link java.util.NoSuchElementException}.
+     * {@link java.lang.IndexOutOfBoundsException}. If the {@link Observable} is empty, {@link java.util.concurrent.Future}
+     * will receive an {@link java.util.NoSuchElementException}. The {@code Observable} source has to terminate in order
+     * for the returned {@code Future} to terminate as well.
      * <p>
      * If the {@code Observable} may emit more than one item, use {@code Observable.toList().toFuture()}.
-     * <p>
-     * <img width="640" height="389" src="https://github.com/ReactiveX/RxJava/wiki/images/rx-operators/toFuture.o.png" alt="">
      * <dl>
      *  <dt><b>Scheduler:</b></dt>
      *  <dd>{@code toFuture} does not operate by default on a particular {@link Scheduler}.</dd>