Update Natspec format to correct devdoc return parameters

[shoenseiwaso]

Update the Natspec documentation to match what solc outputs for devdoc return parameters.

Consider the following minimal smart contract with documented Natspec devdoc return parameters:

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

contract ReturnsDocs {
    /// @return A number
    function returnOne() public pure returns (uint) {
        return 1;
    }

    /// @return A number
    /// @return A string
    function returnTwo() public pure returns (uint, string memory) {
        return (2, "two");
    }
}

Compiling with devdoc output:

$ solc --combined-json devdoc ReturnDocs.sol | jq

{
  "contracts": {
    "ReturnDocs.sol:ReturnsDocs": {
      "devdoc": {
        "kind": "dev",
        "methods": {
          "returnOne()": {
            "returns": {
              "_0": "A number"
            }
          },
          "returnTwo()": {
            "returns": {
              "_0": "A number",
              "_1": "A string"
            }
          }
        },
        "version": 1
      }
    }
  },
  "version": "0.8.26+commit.8a97fa7a.Darwin.appleclang"
}

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