Document size of bool

[nirbheek]

The 2018 edition in git does not document the size of the bool type yet:

https://github.com/rust-lang/book/blob/master/2018-edition/src/ch03-02-data-types.md#the-boolean-type

The size is documented as 1 byte in the stdlib: rust-lang/rust#46156