docs/manual: small fixes and enhancements to adding generic packages

Add overall description of LIBFOO_DEVICES and LIBFOO_PERMISSIONS.
Fix line numbers.

Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>