Clarify that RNG parameters are mandatory in SSL

No change here, these were already mandatory, it just wasn't explicit in
the documentation.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

include/mbedtls/ssl.h

@@ -1535,7 +1535,7 @@ void mbedtls_ssl_conf_verify( mbedtls_ssl_config *conf,
  * \brief          Set the random number generator callback
  *
  * \param conf     SSL configuration
- * \param f_rng    RNG function
+ * \param f_rng    RNG function (mandatory)
  * \param p_rng    RNG parameter
  */
 void mbedtls_ssl_conf_rng( mbedtls_ssl_config *conf,

include/mbedtls/ssl_ticket.h

@@ -90,7 +90,7 @@ void mbedtls_ssl_ticket_init( mbedtls_ssl_ticket_context *ctx );
  * \brief           Prepare context to be actually used
  *
  * \param ctx       Context to be set up
- * \param f_rng     RNG callback function
+ * \param f_rng     RNG callback function (mandatory)
  * \param p_rng     RNG callback context
  * \param cipher    AEAD cipher to use for ticket protection.
  *                  Recommended value: MBEDTLS_CIPHER_AES_256_GCM.