os: document that users of Fd should keep f alive

Fixes #9046.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/162680043

Author: rsc

src/os/file_plan9.go

@@ -25,7 +25,8 @@ type file struct {
 	dirinfo *dirInfo // nil unless directory being read
 }
 
-// Fd returns the integer Unix file descriptor referencing the open file.
+// Fd returns the integer Plan 9 file descriptor referencing the open file.
+// The file descriptor is valid only until f.Close is called or f is garbage collected.
 func (f *File) Fd() uintptr {
 	if f == nil {
 		return ^(uintptr(0))

src/os/file_unix.go

@@ -29,6 +29,7 @@ type file struct {
 }
 
 // Fd returns the integer Unix file descriptor referencing the open file.
+// The file descriptor is valid only until f.Close is called or f is garbage collected.
 func (f *File) Fd() uintptr {
 	if f == nil {
 		return ^(uintptr(0))

src/os/file_windows.go

@@ -36,6 +36,7 @@ type file struct {
 }
 
 // Fd returns the Windows handle referencing the open file.
+// The handle is valid only until f.Close is called or f is garbage collected.
 func (file *File) Fd() uintptr {
 	if file == nil {
 		return uintptr(syscall.InvalidHandle)