Document bool cast semantics

[porglezomp]

After a short debate on the IRC over whether true as i32 is guaranteed to be 1, and false as i32 is guaranteed to be 0, I think that the semantics section on casts should explicitly describe what a bool as integer_type cast does.

[est31]

The reference indirectly documents the representation, but the machine types section would deserve documentation of the bool type as well.

[steveklabnik]

The reference lives in its own repo, so I have moved this there: rust-lang/reference#20 Thanks!