Add short descriptions of the modules

Author: JSorngard

src/cache.rs

@@ -1,3 +1,6 @@
+//! This module contains an implementation of a type that functions as a cache of prime numbers
+//! for related computations.
+
 use crate::{primes, Underlying};
 
 // region: Primes<N>

src/generate.rs

@@ -1,3 +1,5 @@
+//! This module contains implementations of prime generation functions.
+
 use core::fmt;
 
 use crate::{sieve, sieve::sieve_segment, Underlying};

src/search.rs

@@ -1,3 +1,5 @@
+//! This module contains implementations of functions that search for primes that neighbour a given number.
+
 use crate::is_prime;
 
 /// Returns the largest prime smaller than `n` if there is one.

src/sieve.rs

@@ -1,3 +1,5 @@
+//! This module contains implementations of prime sieves.
+
 use core::fmt;
 
 use crate::isqrt;