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>