Auto merge of #2502 - IvanUkhov:doc, r=alexcrichton

doc: make the pages’ titles consistent

The titles of some of the pages end with “Cargo Documentation” (_e.g._, [Frequently Asked Questions](http://doc.crates.io/faq.html)) whereas the titles of some other pages do not (_e.g._, [Environment Variables](http://doc.crates.io/environment-variables.html)), which is a bit inconsistent. Perhaps one should either add that ending to all the titles or eliminate it from all of them. This pull request does the latter, which can be changed if needed. I personally think that such long titles are reasonable for the `title` HTML tag but a bit too verbose when displayed on the page.