os: document that users of Fd should keep f alive

Fixes #9046.

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

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))