DownloaderMiddleware docs: Update process_request and minor cleanups.

This is an update/re-working of https://github.com/scrapy/scrapy/pull/405. Here, we add the fact that `process_request` can raise `IgnoreRequest` in the head paragraph of its documentation. We also add a paragraph describing what will happen when `process_request` returns a request.

The minor cleanups referenced in the commit comment are a few awkward wordings, making the listing of all the possible returns and raises consistent for each method, and standardizing on refering to requests and responses with lowercase and not uppercase first letters.