Note functions that store the RNG callback in a context

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/ssl.h

@@ -2255,6 +2255,10 @@ void mbedtls_ssl_conf_verify(mbedtls_ssl_config *conf,
 /**
  * \brief          Set the random number generator callback
  *
+ * \note           The callback with its parameter must remain valid as
+ *                 long as there is an SSL context that uses the
+ *                 SSL configuration.
+ *
  * \param conf     SSL configuration
  * \param f_rng    RNG function (mandatory)
  * \param p_rng    RNG parameter

include/mbedtls/ssl_ticket.h

@@ -98,7 +98,9 @@ void mbedtls_ssl_ticket_init(mbedtls_ssl_ticket_context *ctx);
  *
  * \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