Update documentation about optional f_rng parameter

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

include/mbedtls/ecp.h

@@ -813,6 +813,9 @@ int mbedtls_ecp_tls_write_group( const mbedtls_ecp_group *grp,
  *                  intermediate results to prevent potential timing attacks
  *                  targeting these results. We recommend always providing
  *                  a non-NULL \p f_rng. The overhead is negligible.
+ *                  Note: unless #MBEDTLS_ECP_NO_INTERNAL_RNG is defined, when
+ *                  \p f_rng is NULL, an internal RNG (seeded from the value
+ *                  of \p m) will be used instead.
  *
  * \param grp       The ECP group to use.
  *                  This must be initialized and have group parameters