runtime: document that Caller and Frame.File always use forward slashes

runtime: document that Caller and Frame.File always use forward slashes

Document that Caller and Frame.File always use forward slashes
as path separators, even on Windows.

Fixes #3335

Change-Id: Ic5bbf8a1f14af64277dca4783176cd8f70726b91
Reviewed-on: https://go-review.googlesource.com/c/go/+/603275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>