runtime/pprof: note different between go test -memprofile and WriteHeapProfile

Fixes #65328

Change-Id: I11242be93a95e117a6758ac037e143c3b38aa71c
Reviewed-on: https://go-review.googlesource.com/c/go/+/597980
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: seankhliao

src/runtime/pprof/pprof.go

@@ -44,7 +44,10 @@
 //	        }
 //	        defer f.Close() // error handling omitted for example
 //	        runtime.GC() // get up-to-date statistics
-//	        if err := pprof.WriteHeapProfile(f); err != nil {
+//	        // Lookup("allocs") creates a profile similar to go test -memprofile.
+//	        // Alternatively, use Lookup("heap") for a profile
+//	        // that has inuse_space as the default index.
+//	        if err := pprof.Lookup("allocs").WriteTo(f, 0); err != nil {
 //	            log.Fatal("could not write memory profile: ", err)
 //	        }
 //	    }