Task.WaitAll does not throw on empty array

[DAXaholic]

According to the current documentation for Task.WaitAll

ArgumentException
The tasks argument contains a null element.

-or-

The tasks argument is an empty array.

the following should throw and catch an exception and consequently write ArgumentException thrown to the console.

using System;
using System.Threading.Tasks;

static void Main(string[] args)
{
    try
    {
        Task.WaitAll();
        Console.WriteLine("Should not reach this point");
    }
    catch (ArgumentException)
    {
        Console.WriteLine("ArgumentException thrown");
    }
}

However - just like Task.WaitAny - waiting on an empty array of tasks does not throw an exception.

Therefore, this commit will remove the wrong statements about throwing an exception on empty arrays for Task.WaitAll.

[dnfclas]

All CLA requirements met.

[rpetrusha]

Thank you, @DAXaholic, for contributing to the dotnet/dotnet-api-docs repo and correcting the erroneous exception information. We'll merge your PR now. Your changes should be live on docs.microsoft.com in the next day or two.