runtime: document runtime.Frames better

runtime: document runtime.Frames better

In particular, this says that Frames.Function uniquely identifies a
function within a program. We depend on this in various places that
use runtime.Frames in std, but it wasn't actually written down.

Change-Id: Ie7ede348c17673e11ae513a094862b60c506abc5
Reviewed-on: https://go-review.googlesource.com/41610
Reviewed-by: Ian Lance Taylor <iant@golang.org>