Improve nova-cells documentation

Explain weight_* features

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>

Change-Id: I9b80c6a9b028e09dbbddab41a1e5c735e682b900
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>