docs: clarify AttributeConstraint works on ICustomAttributeProvider, not arbitrary instances

[Copilot]

The docs incorrectly stated that AttributeConstraint and AttributeExistsConstraint work on "both Type objects and instances (testing the instance's type)." In reality, they work on any object implementing ICustomAttributeProvider — passing an arbitrary class instance that doesn't implement it throws an ArgumentException.

Changes

• AttributeConstraint.md — Replace note 3 with accurate description: the constraint works on ICustomAttributeProvider implementers (Type, MethodInfo, Assembly, etc.), and explicitly notes that passing an arbitrary instance results in an error.
• AttributeExistsConstraint.md — Same correction applied to note 3.

The previously documented behavior would lead users to expect this to work:

// Fails — TestClassWithAttribute does not implement ICustomAttributeProvider
Assert.That(new TestClassWithAttribute(), Has.Attribute<CustomAttribute>());

// Works — typeof() returns a Type, which implements ICustomAttributeProvider
Assert.That(typeof(TestClassWithAttribute), Has.Attribute<CustomAttribute>());

// Also works — MethodInfo implements ICustomAttributeProvider
Assert.That(GetType().GetMethod(nameof(MyTest)), Has.Attribute<TestAttribute>());