Document signal "request_scheduled"

https://github.com/scrapy/scrapy/blob/ad11fc01cb24bae257a153506ce3e40963254074/scrapy/signals.py#L14
https://github.com/scrapy/scrapy/blob/ad11fc01cb24bae257a153506ce3e40963254074/scrapy/core/engine.py#L180-L181