Document that the feature fast_test affects is_prime also in its docs (#88)

* Document that the feature affects is_prime also in its docs

* cargo fmt

Author: JSorngard

src/check.rs

@@ -7,6 +7,8 @@ use crate::integer_math::{mod_mul, mod_pow};
 /// Does trial division with a small wheel up to `log2(n)` and then uses a
 /// deterministic Miller-Rabin primality test.
 ///
+/// If the `fast_test` feature is enabled this function instead calls [`machine_prime::is_prime`] with the `small` feature.
+///
 /// # Example
 ///
 /// Basic usage: