Add missing onSubscribe null-checks to NPE docs on Flowable/Observable subscribe (#6301)

Happened to notice these today

Author: ZacSweers

src/main/java/io/reactivex/Flowable.java

@@ -14461,7 +14461,8 @@ public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super T
      * @throws NullPointerException
      *             if {@code onNext} is null, or
      *             if {@code onError} is null, or
-     *             if {@code onComplete} is null
+     *             if {@code onComplete} is null, or
+     *             if {@code onSubscribe} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
     @CheckReturnValue

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

@@ -12119,7 +12119,8 @@ public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super T
      * @throws NullPointerException
      *             if {@code onNext} is null, or
      *             if {@code onError} is null, or
-     *             if {@code onComplete} is null
+     *             if {@code onComplete} is null, or
+     *             if {@code onSubscribe} is null
      * @see <a href="http://reactivex.io/documentation/operators/subscribe.html">ReactiveX operators documentation: Subscribe</a>
      */
     @CheckReturnValue