Document that Equal is called even if x or y is nil (#63)
diff --git a/cmp/compare.go b/cmp/compare.go
index 143407c..7e215f2 100644
--- a/cmp/compare.go
+++ b/cmp/compare.go
@@ -61,8 +61,8 @@
//
// • If the values have an Equal method of the form "(T) Equal(T) bool" or
// "(T) Equal(I) bool" where T is assignable to I, then use the result of
-// x.Equal(y). Otherwise, no such method exists and evaluation proceeds to
-// the next rule.
+// x.Equal(y) even if x or y is nil.
+// Otherwise, no such method exists and evaluation proceeds to the next rule.
//
// • Lastly, try to compare x and y based on their basic kinds.
// Simple kinds like booleans, integers, floats, complex numbers, strings, and
