reflect: document that Values can't be compared directly

Fixes #9504.

Change-Id: I148f407ace3d1b4db3f19fbb8561d1ee6c4c13b3
Reviewed-on: https://go-review.googlesource.com/2273Reviewed-by: Rob Pike <r@golang.org>

src/reflect/value.go

@@ -30,6 +30,10 @@ const cannotSet = "cannot set value obtained from unexported struct field"
 // A Value can be used concurrently by multiple goroutines provided that
 // the underlying Go value can be used concurrently for the equivalent
 // direct operations.
+//
+// Using == on two Values does not compare the underlying values
+// they represent, but rather the contents of the Value structs.
+// To compare two Values, compare the results of the Interface method.
 type Value struct {
 	// typ holds the type of the value represented by a Value.
 	typ *rtype