cmac: improve parameter validation documentation
diff --git a/include/mbedtls/cmac.h b/include/mbedtls/cmac.h
index f0316e5..e911035 100644
--- a/include/mbedtls/cmac.h
+++ b/include/mbedtls/cmac.h
@@ -100,7 +100,7 @@
* This must be initialized.
* \param input The buffer holding the input data. This must be a
* readable buffer of length \p ilen Bytes. It may be
- * \c NULL if ilen == 0.
+ * \c NULL if `ilen == 0`.
* \param ilen The length of the input data.
*
* \return \c 0 on success.
@@ -121,7 +121,8 @@
* \param ctx The cipher context used for the CMAC operation.
* This must be initialized.
* \param output The output buffer for the CMAC checksum result.
- * This must not be \c NULL.
+ * This must be a writable buffer of length equal to
+ * at least the block-size of the underlying cipher.
*
* \return \c 0 on success.
* \return #MBEDTLS_ERR_MD_BAD_INPUT_DATA
@@ -163,10 +164,11 @@
* \param keylen The length of the CMAC key in bits.
* \param input The buffer holding the input data. This must be a
* readable buffer of length \p ilen Bytes. It may be
- * \c NULL if ilen == 0.
+ * \c NULL if `ilen == 0`.
* \param ilen The length of the input data.
* \param output The buffer for the generic CMAC result.
- * This must not be \c NULL.
+ * This must be a writable buffer of length equal to
+ * at least the block-size of the underlying cipher.
*
* \return \c 0 on success.
* \return #MBEDTLS_ERR_MD_BAD_INPUT_DATA
@@ -191,7 +193,8 @@
* \param input The buffer holding the input data. This must not be \c NULL.
* \param in_len The length of the input data in Bytes.
* \param output The buffer holding the generated 16 Bytes of
- * pseudorandom output. This must not be \c NULL.
+ * pseudorandom output. This must be a writable buffer of size
+ * \c 16 Bytes.
*
* \return \c 0 on success.
*/