cast_ptr_alignment should include a note with the alignment of types involved

[gnzlbg]

Playground:

fn main() {
    unsafe {
        use std::mem::transmute;
        let a = 0_u8;
        let b = &a as *const u8 as *const u64;
        //let _c: &u64 = transmute(b);
        let _d = *b;
    }
}

makes cast_ptr_alignment report

error: casting from `*const u8` to a more-strictly-aligned pointer (`*const u64`)

We might want to add a note stating what the alignment of u8 and of u64 is.