net/http/pprof: mention mutex profile in doc

mutex profile requires explicit calls to
runtime.SetMutexProfileFraction to enable/disable
profiling (like block profile). It is worth
mentioning in the doc.

Change-Id: I2b8d654be9f5c6bc49fc802b7708c4c552fea9b2
Reviewed-on: https://go-review.googlesource.com/42070Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/pprof/pprof.go

@@ -37,6 +37,11 @@
 //
 //	wget http://localhost:6060/debug/pprof/trace?seconds=5
 //
+// Or to look at the holders of contended mutexes, after calling
+// runtime.SetMutexProfileFraction in your program:
+//
+//	go tool pprof http://localhost:6060/debug/pprof/mutex
+//
 // To view all available profiles, open http://localhost:6060/debug/pprof/
 // in your browser.
 //