Update the documentation accordingly. Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com> [yann.morin.1998@free.fr: slight rephrasing in error message, update manual] Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>