The runtime no longer hard-codes the offset of
reflect.methodValue.stack, so remove these obsolete comments. Also,
reflect.methodValue and runtime.reflectMethodValue must also agree
with reflect.makeFuncImpl, so update the comments on all three to
mention this.

This was pointed out by Minux on CL 31138.

Change-Id: Ic5ed1beffb65db76aca2977958da35de902e8e58
Reviewed-on: https://go-review.googlesource.com/34590Reviewed-by: Keith Randall <khr@golang.org>