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/lib.rs

@@ -1,6 +1,7 @@
 #![no_std]
 #![allow(clippy::new_without_default)]
 #![doc = include_str!("../README.md")]
+#![warn(missing_docs)]
 
 // This mod MUST go first, so that the others see its macros.
 pub(crate) mod fmt;