Add explanation for okm_string size

Add explanation for why the size of `okm_string` buffer
is twice as `okm` buffer.

tests/suites/test_suite_hkdf.function

@@ -19,6 +19,10 @@ void test_hkdf( int md_alg, char *hex_ikm_string, char *hex_salt_string,
     unsigned char info[128] = { '\0' };
     unsigned char expected_okm[128] = { '\0' };
     unsigned char okm[128] = { '\0' };
+    /*
+     * okm_string is the string representation of okm,
+     * so its size is twice as the size of okm.
+     */
     unsigned char okm_string[256] = { '\0' };
 
     const mbedtls_md_info_t *md = mbedtls_md_info_from_type( md_alg );