os: point readers at the exec package

R=golang-dev, evan, rsc
CC=golang-dev
https://golang.org/cl/4802046

src/pkg/os/exec_posix.go

@@ -21,6 +21,9 @@ func (sig UnixSignal) String() string {
 
 // StartProcess starts a new process with the program, arguments and attributes
 // specified by name, argv and attr.
+//
+// StartProcess is a low-level interface. The exec package provides
+// higher-level interfaces.
 func StartProcess(name string, argv []string, attr *ProcAttr) (p *Process, err Error) {
 	sysattr := &syscall.ProcAttr{
 		Dir: attr.Dir,
@@ -49,7 +52,9 @@ func (p *Process) Kill() Error {
 // Exec replaces the current process with an execution of the
 // named binary, with arguments argv and environment envv.
 // If successful, Exec never returns.  If it fails, it returns an Error.
-// StartProcess is almost always a better way to execute a program.
+//
+// To run a child process, see StartProcess (for a low-level interface)
+// or the exec package (for higher-level interfaces).
 func Exec(name string, argv []string, envv []string) Error {
 	if envv == nil {
 		envv = Environ()