message: Use better JSON key names in block coinstake output
  Also add help description for new keys
diff: @@ -164,8 +164,8 @@ UniValue blockToJSON(const CBlock& block, const CBlockIndex* blockindex, bool tx
  throw JSONRPCError(RPC_INTERNAL_ERROR, "Cannot get proof of stake hash");
  UniValue stakeData(UniValue::VOBJ);
  - stakeData.push_back(Pair("CS block from id", stake.get()->GetIndexFrom()->GetBlockHash().GetHex()));
  - stakeData.push_back(Pair("CS block from height", stake.get()->GetIndexFrom()->nHeight));
  + stakeData.push_back(Pair("BlockFromHash", stake.get()->GetIndexFrom()->GetBlockHash().GetHex()));
  + stakeData.push_back(Pair("BlockFromHeight", stake.get()->GetIndexFrom()->nHeight));
  stakeData.push_back(Pair("hashProofOfStake", hashProofOfStakeRet.GetHex()));
  stakeData.push_back(Pair("stakeModifierHeight", ((stake->IsZPIV()) ? "Not available" : std::to_string(
  stake->getStakeModifierHeight()))));
  @@ -610,6 +610,12 @@ UniValue getblock(const UniValue& params, bool fHelp)
  " \"1000\" : n, (numeric) supply of 1000 zPIV denomination\n"
  " \"5000\" : n, (numeric) supply of 5000 zPIV denomination\n"
  " \"total\" : n, (numeric) The total supply of all zPIV denominations\n"
  + " },\n"
  + " \"CoinStake\" :\n"
  + " \"BlockFromHash\" : \"hash\", (string) Block hash of the coin stake input\n"
  + " \"BlockFromHeight\" : n, (numeric) Block Height of the coin stake input\n"
  + " \"hashProofOfStake\" : \"hash\", (string) Proof of Stake hash\n"
  + " \"stakeModifierHeight\" : \"nnn\" (string) Stake modifier block height\n"
  " }\n"
  "}\n"
  