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 <[email protected]>
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: aclements

src/runtime/pprof/proto.go

@@ -395,6 +395,10 @@ func (b *profileBuilder) build() {
 // location ID slice, locs. The addresses in the stack are return PCs or 1 + the PC of
 // an inline marker as the runtime traceback function returns.
 //
+// It may return an empty slice even if locs is non-empty, for example if locs consists
+// solely of runtime.goexit. We still count these empty stacks in profiles in order to
+// get the right cumulative sample count.
+//
 // It may emit to b.pb, so there must be no message encoding in progress.
 func (b *profileBuilder) appendLocsForStack(locs []uint64, stk []uintptr) (newLocs []uint64) {
 	b.deck.reset()