runtime/pprof: document possibility of empty stacks
I spent quite a while determining the cause of empty stacks in
profiles and reasoning out why this is okay. There isn't a great place
to record this knowledge, but a documentation comment on
appendLocsForStack is better than nothing.
Updates #51550.
Change-Id: I2eefc6ea31f1af885885c3d96199319f45edb4ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/460695
Reviewed-by: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>