Document behavioral differences with fluentassertions

[vbreuss]

1. When checking "contains at most" against null, it throws:

   string actual = null;
   // Does not throw with fluentassertions, but does with aweXpect
   Expect.That(actual).Contains(expectedSubstring).AtMost(1.Times());
   Expect.That(actual).Contains(expectedSubstring).LessThan(1.Times());

2. All().Be does not mix type and instances (see AllBeAssignableTo behaving not like foreach BeAssignableTo fluentassertions/fluentassertions#1006)

   var collection = new object[] { typeof(int), 2, typeof(int) };
   collection.Should().AllBeAssignableTo<int>(); // Succeeds with fluentassertions
   await Expect.That(collection).All().Are<int>(); // Fails with aweXpect

3. DateTime with different Kind is considered different in aweXpect

   var actual = new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Local);
   var expected = new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc);
   actual.Should().Be(expected); // Succeeds with fluentassertions
   await Expect.That(actual).IsEqualTo(expected); // Fails with aweXpect

4. automatic flattening of AggregateException:

   Func<Task> act = () => throw new AggregateException(new ArgumentException("It is nested within an `AggregateException`"));
   act.Should().Throw<ArgumentException>(); // Succeeds with fluentassertions
   await Expect.That(act).Throws<ArgumentException>(); // Fails with aweXpect

5. BeInRange for numbers throws an XUnitException instead of ArgumentException when minimum or maximum is NaN.
   Same for IsEqualTo("").AsWildcard() (which throws an ArgumentException on fluentassertions)