Add docstring to reexec.Command

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

reexec/command_unsupported.go

@@ -6,6 +6,7 @@ import (
 	"os/exec"
 )
 
+// Command is unsupported on operating systems apart from Linux and Windows.
 func Command(args ...string) *exec.Cmd {
 	return nil
 }