docs: document all public apis of embedded-hal-internal

* Make some fields and functions non-public where possible.
* Enable doc warnings for missing public API docs.

embassy-hal-internal/src/ratio.rs

@@ -1,3 +1,4 @@
+//! Types for dealing with rational numbers.
 use core::ops::{Add, Div, Mul};
 
 use num_traits::{CheckedAdd, CheckedDiv, CheckedMul};