Push count <= 8 precondition into doc comment.

The alternative would be to actually convert the `debug_assert` to
an `assert`, or clamp to 8, but both of these could conceivably
have a performance impact, so in the interest of being a pure
win, with no downsides, that change is left to a future editor.

arrow-buffer/src/util/bit_mask.rs

@@ -127,7 +127,7 @@ unsafe fn set_upto_64bits(
 }
 
 /// # Safety
-/// The caller must ensure `data` has `offset..(offset + 8)` range
+/// The caller must ensure `data` has `offset..(offset + 8)` range, and `count <= 8`.
 #[inline]
 unsafe fn read_bytes_to_u64(data: &[u8], offset: usize, count: usize) -> u64 {
     debug_assert!(count <= 8);