os/exec: document that LookPath's result may be a relative path

Fixes #3622

R=golang-dev, alex.brainman
CC=golang-dev
https://golang.org/cl/12971043

Author: robpike

src/pkg/os/exec/lp_plan9.go

@@ -28,6 +28,7 @@ func findExecutable(file string) error {
 // in the directories named by the path environment variable.
 // If file begins with "/", "#", "./", or "../", it is tried
 // directly and the path is not consulted.
+// The result may be an absolute path or a path relative to the current directory.
 func LookPath(file string) (string, error) {
 	// skip the path lookup for these prefixes
 	skip := []string{"/", "#", "./", "../"}

src/pkg/os/exec/lp_unix.go

@@ -29,6 +29,7 @@ func findExecutable(file string) error {
 // LookPath searches for an executable binary named file
 // in the directories named by the PATH environment variable.
 // If file contains a slash, it is tried directly and the PATH is not consulted.
+// The result may be an absolute path or a path relative to the current directory.
 func LookPath(file string) (string, error) {
 	// NOTE(rsc): I wish we could use the Plan 9 behavior here
 	// (only bypass the path if file begins with / or ./ or ../)

src/pkg/os/exec/lp_windows.go

@@ -47,6 +47,7 @@ func findExecutable(file string, exts []string) (string, error) {
 // If file contains a slash, it is tried directly and the PATH is not consulted.
 // LookPath also uses PATHEXT environment variable to match
 // a suitable candidate.
+// The result may be an absolute path or a path relative to the current directory.
 func LookPath(file string) (f string, err error) {
 	x := os.Getenv(`PATHEXT`)
 	if x == `` {