runtime: document that tracefpunwindoff applies to some profilers

The tracefpunwindoff GODEBUG applies to all the places where we use
frame pointer unwinding. Originally that was just the execution tracer,
but now we also use it for the block and mutex profilers.

Change-Id: I6a6a69641fede668d96335582dba6d43e62f6a51
Reviewed-on: https://go-review.googlesource.com/c/go/+/718522
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Nick Ripley <nick.ripley@datadoghq.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

Author: nsrip-dd

src/runtime/extern.go

@@ -211,10 +211,11 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	report. This also extends the information returned by runtime.Stack.
 	Setting N to 0 will report no ancestry information.
 
-	tracefpunwindoff: setting tracefpunwindoff=1 forces the execution tracer to
-	use the runtime's default stack unwinder instead of frame pointer unwinding.
-	This increases tracer overhead, but could be helpful as a workaround or for
-	debugging unexpected regressions caused by frame pointer unwinding.
+	tracefpunwindoff: setting tracefpunwindoff=1 forces the execution tracer
+	and block and mutex profilers to use the runtime's default stack
+	unwinder instead of frame pointer unwinding. This increases their
+	overhead, but could be helpful as a workaround or for debugging
+	unexpected regressions caused by frame pointer unwinding.
 
 	traceadvanceperiod: the approximate period in nanoseconds between trace generations. Only
 	applies if a program is built with GOEXPERIMENT=exectracer2. Used primarily for testing