os: document the type of link errors

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5673090

src/pkg/os/file_plan9.go

@@ -276,7 +276,6 @@ func Remove(name string) error {
 }
 
 // Rename renames a file.
-// If there is an error, it will be of type *PathError.
 func Rename(oldname, newname string) error {
 	var d Dir
 	d.Null()
@@ -340,12 +339,15 @@ func Pipe() (r *File, w *File, err error) {
 // not supported on Plan 9
 
 // Link creates a hard link.
+// If there is an error, it will be of type *LinkError.
 func Link(oldname, newname string) error {
-	return ErrPlan9
+	return &LinkError{"link", oldname, newname, ErrPlan9}
 }
 
+// Symlink creates newname as a symbolic link to oldname.
+// If there is an error, it will be of type *LinkError.
 func Symlink(oldname, newname string) error {
-	return ErrPlan9
+	return &LinkError{"symlink", oldname, newname, ErrPlan9}
 }
 
 func Readlink(name string) (string, error) {

src/pkg/os/file_posix.go

@@ -38,6 +38,7 @@ func (e *LinkError) Error() string {
 }
 
 // Link creates newname as a hard link to the oldname file.
+// If there is an error, it will be of type *LinkError.
 func Link(oldname, newname string) error {
 	e := syscall.Link(oldname, newname)
 	if e != nil {
@@ -47,6 +48,7 @@ func Link(oldname, newname string) error {
 }
 
 // Symlink creates newname as a symbolic link to oldname.
+// If there is an error, it will be of type *LinkError.
 func Symlink(oldname, newname string) error {
 	e := syscall.Symlink(oldname, newname)
 	if e != nil {