Fix documentation for `slice()`

Fixes https://github.com/mozilla/rust/issues/14577
Document BigInt's new and from_slice methods

Fixes https://github.com/mozilla/rust/issues/14639
auto merge of #14668 : aochagavia/rust/pr3, r=alexcrichton

Closes https://github.com/mozilla/rust/issues/14577
Closes https://github.com/mozilla/rust/issues/14639