docs/manual: documentation for recursive makedevs syntax

This patch adds the documentation for the new makedevs
recursive syntax.

[Thomas: s/folder/directory/ as suggested by Yann.]

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>