rpc: update the doc for data field in outputs argument

This affects docs of the following RPCs:
`bumpfee`, `psbtbumpfee`, `send`, `walletcreatefundedpsbt`, `createpsbt`,
and `createrawtransaction`

It was not evident to me that this field creates an `OP_RETURN` output until
I read the code and tried it out. Thus, making the doc explicitly mention it.

src/rpc/rawtransaction.cpp

@@ -150,7 +150,7 @@ static std::vector<RPCArg> CreateTxDoc()
                 },
                 {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
                     {
-                        {"data", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "A key-value pair. The key must be \"data\", the value is hex-encoded data"},
+                        {"data", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "A key-value pair. The key must be \"data\", the value is hex-encoded data that becomes a part of an OP_RETURN output"},
                     },
                 },
             },

src/wallet/rpc/spend.cpp

@@ -978,7 +978,7 @@ static std::vector<RPCArg> OutputsDoc()
         },
         {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
             {
-                {"data", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "A key-value pair. The key must be \"data\", the value is hex-encoded data"},
+                {"data", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "A key-value pair. The key must be \"data\", the value is hex-encoded data that becomes a part of an OP_RETURN output"},
             },
         },
     };