docs: update safety comment

fr32/src/reader.rs

@@ -68,6 +68,9 @@ impl<R: Read> Fr32Reader<R> {
     fn process_block(&mut self) {
         let in_buffer: &[u128] = {
             #[cfg(target_arch = "aarch64")]
+            // Safety: This is safe because the struct is aligned on
+            // a 16 byte boundary and can therefore be casted from u128
+            // to u8 without alignment safety issues.
             unsafe {
                 &mut (*(&self.in_buffer as *const [u8] as *mut [u128]))
             }