os: document that NewFile can return nil

Fixes #20023

Change-Id: I1bec3e69031ffcfd7ad71716be9597ec322528ff
Reviewed-on: https://go-review.googlesource.com/41211
Reviewed-by: Russ Cox <rsc@golang.org>

src/os/file_unix.go

@@ -59,7 +59,9 @@ func (f *File) Fd() uintptr {
 	return uintptr(f.pfd.Sysfd)
 }
 
-// NewFile returns a new File with the given file descriptor and name.
+// NewFile returns a new File with the given file descriptor and
+// name. The returned value will be nil if fd is not a valid file
+// descriptor.
 func NewFile(fd uintptr, name string) *File {
 	return newFile(fd, name, false)
 }