Update documentation

src/main/java/com/spotify/futures/ConcurrencyLimiter.java

@@ -70,6 +70,8 @@ public static <T> ConcurrencyLimiter<T> create(int maxConcurrency, int maxQueueS
    * @param callable - a function that creates a future.
    * @returns a proxy future that completes with the future created by the
    *          input function.
+   * @throws {@link NullPointerException} if callable is null
+   * @throws {@link CapacityReachedException} when soft queue size limit is exceeded.
    */
   public ListenableFuture<T> add(Callable<? extends ListenableFuture<T>> callable) {
     Preconditions.checkNotNull(callable);