Note functions that store the RNG callback in a context

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/ssl_ticket.h b/include/mbedtls/ssl_ticket.h
index bf3bc76..9f7e440 100644
--- a/include/mbedtls/ssl_ticket.h
+++ b/include/mbedtls/ssl_ticket.h
@@ -98,7 +98,9 @@
  *
  * \param ctx       Context to be set up
  * \param f_rng     RNG callback function (mandatory)
- * \param p_rng     RNG callback context
+ * \param p_rng     RNG callback context.
+ *                  Note that the RNG callback must remain valid
+ *                  until the ticket context is freed.
  * \param cipher    AEAD cipher to use for ticket protection.
  *                  Recommended value: MBEDTLS_CIPHER_AES_256_GCM.
  * \param lifetime  Tickets lifetime in seconds