os/exec: status of FDs >= len(cmd.ExtraFiles)+3 is undocumented

What version of Go are you using (go version)?

go version go1.14 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64" GOOS="linux"

What did you do?

Invocation: $ parent-cmd 0<a 1>b 2>c 3>d 4>e 5>f

cmd := exec.Command("child-cmd")
cmd.ExtraFiles = []*os.File{os.Stderr}
cmd.Stdin = os.Stdin
cmd.Stdout = os.Stdout
cmd.Stderr = os.Stderr
cmd.Start() // in child-cmd: 0=a 1=b 2=c 3=c 4=e 5=closed!

What did you expect to see?

File descriptors 4 and 5 (>= len(cmd.ExtraFiles)+3) to be either both inherited or none, and the behavior to be documented.

What did you see instead?

File descriptor 4 is inherited, but 5 is not. The doc doesn't say anything about FDs >= len(cmd.ExtraFiles)+3 in the child process.