doc: contribution guidelines: Clarify and extend

Clarify and extend some of the PR and contribution guidelines so that
they cover practices that have been effectively enforced by maintainers,
but were never properly documented.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>