Specify AES-CMAC-PRF buffer output size in comment
diff --git a/include/mbedtls/cmac.h b/include/mbedtls/cmac.h
index 325d22d..ddacb21 100644
--- a/include/mbedtls/cmac.h
+++ b/include/mbedtls/cmac.h
@@ -120,7 +120,7 @@
* \param key_len PRF key length
* \param input buffer holding the input data
* \param in_len length of the input data in bytes
- * \param tag buffer holding the generated pseudorandom output
+ * \param tag buffer holding the generated pseudorandom output (16 bytes)
*
* \return 0 if successful
*/