Improve LMS private function warning

Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/include/mbedtls/lms.h b/include/mbedtls/lms.h
index 15a1a1b..850bb6a 100644
--- a/include/mbedtls/lms.h
+++ b/include/mbedtls/lms.h
@@ -347,7 +347,8 @@
  *
  * \warning                  This function is **not intended for use in
  *                           production**, due to as-yet unsolved problems with
- *                           handling stateful keys.
+ *                           handling stateful keys. The API for this function
+ *                           may change considerably in future versions.
  *
  * \note                     The seed must have at least 256 bits of entropy.
  *
@@ -398,7 +399,8 @@
  *
  * \warning                  This function is **not intended for use in
  *                           production**, due to as-yet unsolved problems with
- *                           handling stateful keys.
+ *                           handling stateful keys. The API for this function
+ *                           may change considerably in future versions.
  *
  * \note                     Before this function is called, the context must
  *                           have been initialized and must contain a private