Rollup merge of #24510 - fhartwig:broken-links, r=nikomatsakis

 Fix broken links in various parts of the docs.
I also found a dead link [here](http://doc.rust-lang.org/nightly/alloc/boxed/) (the first link on the page), but the chapter of the book that it used to point at seems to be gone, and I'm not sure what should happen to that link.

Author: Manishearth