rpc: Document an RPCResult for all calls; Enforce at compile time

src/rpc/mining.cpp

@@ -751,7 +751,7 @@ static UniValue submitblock(const JSONRPCRequest& request)
                     {"hexdata", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "the hex-encoded block data to submit"},
                     {"dummy", RPCArg::Type::STR, /* default */ "ignored", "dummy value, for compatibility with BIP22. This value is ignored."},
                 },
-                RPCResults{},
+                RPCResult{RPCResult::Type::NONE, "", "Returns JSON Null when valid, a string according to BIP22 otherwise"},
                 RPCExamples{
                     HelpExampleCli("submitblock", "\"mydata\"")
             + HelpExampleRpc("submitblock", "\"mydata\"")