Document the behavior of `true|false as {integral}`

Currently, it appears that `true as {integral} == 1 && false as {integral} == 0`. Unfortunately, the only documentation on this subject appears to be a tangentially related bullet point in the [reference](https://doc.rust-lang.org/reference.html#behavior-considered-undefined).

It should probably be documented in the [the bool documentation](https://doc.rust-lang.org/std/primitive.bool.html).