net/http/pprof: note calling runtime.SetBlockProfileRate is required for block profile

Fixes #15076

Change-Id: I5ce8f6253245d8cc1f862a1bf618775f557f955d
Reviewed-on: https://go-review.googlesource.com/21610Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/pprof/pprof.go

@@ -30,7 +30,8 @@
 //
 //	go tool pprof http://localhost:6060/debug/pprof/profile
 //
-// Or to look at the goroutine blocking profile:
+// Or to look at the goroutine blocking profile, after calling
+// runtime.SetBlockProfileRate in your program:
 //
 //	go tool pprof http://localhost:6060/debug/pprof/block
 //