Documentation: Update `bt_gatt_discover_params` & `bt_gatt_discover_params`

Documentation for `bt_gatt_subscribe_params` and
`bt_gatt_discover_params` is not complete.
`CONFIG_BT_GATT_AUTO_DISCOVER_CCC` need to be selected so Doxygen add
see `end_handle`, `disc_params` and `sub_params`.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>