Bluetooth: gatt: Update bt_gatt_is_subscribed documentation

This updates the documentation of bt_gatt_is_subscribed function that
can take a bitfield of BT_GATT_CCC_NOTIFY and BT_GATT_CCC_INDICATE.
This might be useful if one wants to test if peer is subscribed, but
does not matter which method was used.

The gatt.c implementation handles API usage already, because it performs
bitwise AND:
if (bt_conn_is_peer_addr_le(conn, cfg->id, &cfg->peer) &&
    (ccc_value & ccc->cfg[i].value)) {
	return true;
}

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>