Add docstring to reexec.Command

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

reexec/command_windows.go

@@ -6,6 +6,9 @@ import (
 	"os/exec"
 )
 
+// Command returns *exec.Cmd which have Path as current binary.
+// For example if current binary is "docker.exe" at "C:\", then cmd.Path will
+// be set to "C:\docker.exe".
 func Command(args ...string) *exec.Cmd {
 	return &exec.Cmd{
 		Path: Self(),