cmd/go: better advice for heap profiling

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/60760048

src/cmd/go/test.go

@@ -168,9 +168,7 @@ control the execution of any test:
 	    Enable more precise (and expensive) memory profiles by setting
 	    runtime.MemProfileRate.  See 'godoc runtime MemProfileRate'.
 	    To profile all memory allocations, use -test.memprofilerate=1
-	    and set the environment variable GOGC=off to disable the
-	    garbage collector, provided the test can run in the available
-	    memory without garbage collection.
+	    and pass --alloc_space flag to pprof tool.
 
 	-outputdir directory
 	    Place output files from profiling in the specified directory,