cmp: clarify when a nil pointer reaches an Equal method

[Sanjays2402]

Fixes #363.

The Equal documentation says a user-defined Equal method is used "even if x or y is nil". #363 reports this is not what happens. Both are partly right, and the difference is where the method is declared.

comparePtr short-circuits on a nil operand before dereferencing:

func (s *state) comparePtr(t reflect.Type, vx, vy reflect.Value) {
	if vx.IsNil() || vy.IsNil() {
		s.report(vx.IsNil() && vy.IsNil(), 0)
		return
	}

A pointer is dereferenced before the method is considered, so a nil *T never reaches an Equal method declared on T. When the method is declared on *T, it is found on the pointer itself and the documented behavior holds.

Observed with the reproducer from the issue:

Equal declared on	x=nil, y=non-nil	method called
value type T	false	no
pointer type *T	true	yes

The behavior looks correct to me and changing it would give up nil-safety, so this only adjusts the documentation and adds a regression test covering both receiver forms.

Tested with go test ./... on go1.26.3 (darwin/arm64); all packages pass. I also flipped one expectation to confirm the new test fails when the behavior changes.