issue #403: add documentation note in api.rst

[atranou]

The priority is ignored when multiple priorities have been inserted as parameters. This happens because of the line 45 inwebservice.py, where all the arguments are affected except from setting So this curl http://localhost:6800/schedule.json -d project=project -d spider=spider_name -d priority=3 -d priority=2 is equal to this curl http://localhost:6800/schedule.json -d project=project -d spider=spider_name -d priority=3