Mention the Montgomery curve exception

Montgomery curves are not in the expected place in the curve list.
This is a bug (https://github.com/ARMmbed/mbedtls/issues/4698), but
until this bug is fixed, document the current behavior and indicate
that it's likely to change.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index bd0f5d7..30dfbce 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -2987,7 +2987,9 @@
 #if defined(MBEDTLS_ECP_C)
 /**
  * \brief          Set the allowed curves in order of preference.
- *                 (Default: all defined curves in order of decreasing size.)
+ *                 (Default: all defined curves in order of decreasing size,
+ *                 except that Montgomery curves come last. This order
+ *                 is likely to change in a future version.)
  *
  *                 On server: this only affects selection of the ECDHE curve;
  *                 the curves used for ECDH and ECDSA are determined by the