fix various issues.

- comments issues
- code format style issues
- naming improvement.
- error return improvements

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_keys.h b/library/ssl_tls13_keys.h
index ea553e3..b1155fb 100644
--- a/library/ssl_tls13_keys.h
+++ b/library/ssl_tls13_keys.h
@@ -700,8 +700,9 @@
  * \param[out]  psk  PSK output pointer.
  * \param[out]  psk_len Length of PSK.
  *
- * \returns    \c 0 if there is configured PSK and exported success.
- * \returns    A negative error code on failure.
+ * \returns     \c 0 if there is a configured PSK and it was exported
+ *              successfully.
+ * \returns     A negative error code on failure.
  */
 MBEDTLS_CHECK_RETURN_CRITICAL
 int mbedtls_ssl_tls13_export_handshake_psk( mbedtls_ssl_context *ssl,