DOWNLOAD_DELAY docs clarification

* delay is enforced per website, not per spider;
* document `download_delay` attribute (it was previously documented only in FAQ about 999 error codes);
* document how `CONCURRENT_REQUESTS_PER_IP` affects download delays.

I'm not sure `CONCURRENT_REQUESTS_PER_IP` is an intended feature; I got an idea that it works this way by reading https://github.com/scrapy/scrapy/blob/master/scrapy/core/downloader/__init__.py.