2.x: Add note about NoSuchElementException to Single.zip(). (#5876)

* 2.x: Add note about NoSuchElementException to Single.zip().

* Address code review comments.

Author: artem-zinnatullin

src/main/java/io/reactivex/Single.java

@@ -1279,6 +1279,8 @@ public static <T> Single<T> wrap(SingleSource<T> source) {
      * value and calls a zipper function with an array of these values to return a result
      * to be emitted to downstream.
      * <p>
+     * If the {@code Iterable} of {@link SingleSource}s is empty a {@link NoSuchElementException} error is signalled after subscription.
+     * <p>
      * Note on method signature: since Java doesn't allow creating a generic array with {@code new T[]}, the
      * implementation of this operator has to create an {@code Object[]} instead. Unfortunately, a
      * {@code Function<Integer[], R>} passed to the method would trigger a {@code ClassCastException}.
@@ -1294,7 +1296,8 @@ public static <T> Single<T> wrap(SingleSource<T> source) {
      * </dl>
      * @param <T> the common value type
      * @param <R> the result value type
-     * @param sources the Iterable sequence of SingleSource instances
+     * @param sources the Iterable sequence of SingleSource instances. An empty sequence will result in an
+     *                {@code onError} signal of {@link NoSuchElementException}.
      * @param zipper the function that receives an array with values from each SingleSource
      *               and should return a value to be emitted to downstream
      * @return the new Single instance
@@ -1721,6 +1724,8 @@ public static <T1, T2, T3, T4, T5, T6, T7, T8, T9, R> Single<R> zip(
      * value and calls a zipper function with an array of these values to return a result
      * to be emitted to downstream.
      * <p>
+     * If the array of {@link SingleSource}s is empty a {@link NoSuchElementException} error is signalled immediately.
+     * <p>
      * Note on method signature: since Java doesn't allow creating a generic array with {@code new T[]}, the
      * implementation of this operator has to create an {@code Object[]} instead. Unfortunately, a
      * {@code Function<Integer[], R>} passed to the method would trigger a {@code ClassCastException}.
@@ -1736,7 +1741,8 @@ public static <T1, T2, T3, T4, T5, T6, T7, T8, T9, R> Single<R> zip(
      * </dl>
      * @param <T> the common value type
      * @param <R> the result value type
-     * @param sources the array of SingleSource instances
+     * @param sources the array of SingleSource instances. An empty sequence will result in an
+     *                {@code onError} signal of {@link NoSuchElementException}.
      * @param zipper the function that receives an array with values from each SingleSource
      *               and should return a value to be emitted to downstream
      * @return the new Single instance