docs(doctests+coverage): add documentation and doctests for all modules of secret-memory except for alloc

secret-memory/src/rand.rs

@@ -1,5 +1,9 @@
+//! This module provides functionality for generating random numbers using the [rand] crate.
+
+/// We use the [ThreadRng](rand::rngs::ThreadRng) for randomness in this crate.
 pub type Rng = rand::rngs::ThreadRng;
 
+/// Get the default [Rng].
 pub fn rng() -> Rng {
     rand::thread_rng()
 }