Allow disabling hint on specifically `Assert.NotNull`

[Alxandr]

Description

Assert.NotNull from xunit has annotations that .NET. This method is thus not just to convert in the same vein as most other xunit assertions.

Complete minimal example reproducing the issue

var subject = GetThingOrNull();
Assert.NotNull(subject);
subject.Property.Should().HaveCount(5);

This is fine, but if I change the Assert.NotNull per fluent-assertions suggestion to subject.Should().NotBeNull(), I get a compiler-warning on the property access.