[Docs] ContainItemsAssignableTo

[cheng93]

With the fixes for #1618 the docs need updating.

https://fluentassertions.com/tips/#collections

MSTest	FluentAssertions
CollectionAssert.AllItemsAreInstancesOfType(actual, typeof(T));	actual.Should().ContainItemsAssignableTo<T>();
CollectionAssert.AllItemsAreInstancesOfType failed. Element at index 0 is not of expected type. Expected type:<UnitTests2.MyIdenticalClass>. Actual type:<UnitTests2.MyClass>.	Expected collection to contain only items of type UnitTests2.MyIdenticalClass, but item SomeProperty: 1, OtherProperty: item at index 0 is of type UnitTests2.MyClass.