Add module docstrings to remaining modules

I think it makes the docs look a little nicer when all the top-level
modules have some short description, if only to expand the abbreviations

Author: Svein Kristian Rougseth

src/dac.rs

@@ -1,3 +1,5 @@
+//! Digital-to-analog converter
+
 use crate::pac::DAC;
 use crate::{
     gpio::{

src/ltdc.rs

@@ -1,3 +1,5 @@
+//! Interface to the LCD-TFT display controller
+
 #[cfg_attr(test, allow(unused_imports))]
 use micromath::F32Ext;
 

src/rng.rs

@@ -1,3 +1,5 @@
+//! Interface to the true random number generator
+
 use core::cmp;
 use core::mem;
 

src/serial.rs

@@ -1,3 +1,5 @@
+//! Serial communication using UART/USART peripherals
+
 use core::fmt;
 use core::marker::PhantomData;
 use core::ops::Deref;

src/timer.rs

@@ -1,3 +1,5 @@
+//! Interface to the timer peripherals
+
 #![allow(non_upper_case_globals)]
 
 use core::convert::TryFrom;