Doxygen comments improvement
diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index f68643d..6a49c39 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -143,7 +143,7 @@
* \param iv_len The length of the IV.
* \param add The buffer holding the additional data. If \p add_len is
* greater than zero, this must be of at least that size in
- * Bytes.
+ * Bytes. If `add_len == 0`, this may be \c NULL.
* \param add_len The length of the additional data.
* \param input The buffer holding the input data. If \p length is greater
* than zero, this must be a readable buffer of at least that
@@ -190,7 +190,7 @@
* \param iv_len The length of the IV.
* \param add The buffer holding the additional data. If \p add_len is
* greater than zero, this must be of at least that size in
- * Bytes.
+ * Bytes. If `add_len == 0`, this may be \c NULL.
* \param add_len The length of the additional data.
* \param tag The buffer holding the tag to verify. This must be a
* readable buffer of at least \p tag_len Bytes.
@@ -229,10 +229,10 @@
* \param iv The initialization vector. This must be a readable buffer of
* at least \p iv_len Bytes.
* \param iv_len The length of the IV.
- * \param add The buffer holding the additional data, or NULL
- * if \p add_len is 0.
- * \param add_len The length of the additional data. If 0,
- * \p add may be NULL.
+ * \param add The buffer holding the additional data, or \c NULL
+ * if \p add_len is \c 0.
+ * \param add_len The length of the additional data. If \c 0,
+ * \p add may be \c NULL.
*
* \return \c 0 on success.
*/