rpi: improve documentation

Signed-off-by: Mathieu Benoit <mathieu.benoit@savoirfairelinux.com>
[yann.morin.1998@free.fr: further improvements for readability and
    completeness]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>