reflect: adjust documentation on Value

Make the documentation more explicit that it is not safe to directly
compare Value. Get straight to the point on how to do it correctly.

Updates #18871

Change-Id: I2aa3253f779636b2f72a1aae8c9bb45d3c32c902
Reviewed-on: https://go-review.googlesource.com/36018Reviewed-by: Keith Randall <khr@golang.org>

src/reflect/value.go

@@ -30,9 +30,9 @@ const ptrSize = 4 << (^uintptr(0) >> 63) // unsafe.Sizeof(uintptr(0)) but an ide
 // 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.
+// Using == on two Values does not compare the underlying values
+// they represent.
 type Value struct {
 	// typ holds the type of the value represented by a Value.
 	typ *rtype