cmd/go: add comment about SIGUSR2 on iOS

Missing from CL 34926.

Change-Id: I4a046440c30811f26da53bee0e853dae3b0ac57a
Reviewed-on: https://go-review.googlesource.com/35123Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/test.go

@@ -1531,6 +1531,10 @@ func coverRegisterFile(fileName string, counter []uint32, pos []uint32, numStmts
 
 func main() {
 {{if .IsIOS}}
+	// Send a SIGUSR2, which will be intercepted by LLDB to
+	// tell the test harness that installation was successful,
+	// and to give the exec script a chance set the current
+	// working directory. See misc/ios/go_darwin_arm_exec.go.
 	signal.Notify(make(chan os.Signal), syscall.SIGUSR2)
 	syscall.Kill(0, syscall.SIGUSR2)
 	signal.Reset(syscall.SIGUSR2)