This adds strictly more information to the source files and reduces the
need for customized tooling to render the book.
(While this should not change the output of _rustbook_, it is very
useful when rendering the sources with external tools like Pandoc.)
At https://doc.rust-lang.org/book/vectors.html, there should be a link to
Generics page but the link address is ommitted and thus link is not functioning
well. So I added a link definition to the vectors.md.
r? @steveklabnik
Feel silly because it's just one thing but it was bothering me that the documentation tells you what you're not going to learn now instead of linking to the item in question. Am total noob pls assist, where is power button. :)
