Add rustdoc links in most of the documentation

[alichay]

There are a couple of places where the links may be a little bit ugly in the source, but I opted to keep them when appropriate because the usability improvement on the docs page is pretty nice. The worst offenders have their links put as references at the bottom of the block, but I tried to be sparing with this because it's a bit more annoying to maintain.

While this would probably be enough to close #650, note that this only targets cpal and not the contained asio-sys.

[est31]

Can you run rustfmt? There are failures due to it in CI.

[alichay]

Whoops, there we go 😅

[est31]

Oh sorry I forgot about this PR. Could you rebase it? Then I can merge it :).

[alichay]

Oh sorry I forgot about this PR. Could you rebase it? Then I can merge it :).

mmhm, it should be rebased now!

[Be-ing]

Thanks!