A lot of code that uses runtime.Callers makes assumptions about the
result that are not true today under gccgo and will not be true in the
future in gc. This adds a section to the release notes discussing how
to correctly use runtime.Callers.

Change-Id: I96b7c7ef183cee2061442fc3501fceceefa54c09
Reviewed-on: https://go-review.googlesource.com/47691Reviewed-by: Russ Cox <rsc@golang.org>