docs: document all embassy-rp public apis

Enable missing doc warnings.

embassy-rp/src/lib.rs

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