Clarify requirements on handling ECP group IDs
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 0e5b47f..1c37298 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -159,6 +159,10 @@
* additions or subtractions. Therefore, it is only an approximative modular
* reduction. It must return 0 on success and non-zero on failure.
*
+ * \note Alternative implementations must keep the group IDs distinct. If
+ * two group structures have the same ID, then they must be
+ * identical.
+ *
*/
typedef struct mbedtls_ecp_group
{