[bitshares/bitshares-core] `subscribe_to_market` returned values (#1258)

Also, only have so far, BitShares [operations list](https://github.com/bitshares/dev.bitshares.works/blob/master/core/components/operations.md#bitshares-core—graphenechain) and [objects list](
https://github.com/bitshares/dev.bitshares.works/blob/master/core/components/objects.md#object-names-and-detailed-descriptions) if want to read the Detailed Descriptions.