runtime/pprof: clarify CPU profile's captured during the lifetime of the prog

Fixes #18504.

Change-Id: I3716fc58fc98472eea15ce3617aee3890670c276
Reviewed-on: https://go-review.googlesource.com/36430Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/pprof/pprof.go

@@ -33,7 +33,9 @@
 //            }
 //            defer pprof.StopCPUProfile()
 //        }
-//        ...
+//
+//        // ... rest of the program ...
+//
 //        if *memprofile != "" {
 //            f, err := os.Create(*memprofile)
 //            if err != nil {