doc(methods): add generic documentation on all rpc methods

[miraclx]

Add generic documentation on all methods in near_jsonrpc_client::methods... primarily for clarity. Showing which exported type does what. Disambiguating between Rpc*Request / Rpc*Response & Rpc*Error types.

Example generated documentation

Module near_jsonrpc_client::methods::broadcast_tx_async

Example

use near_jsonrpc_client::{errors, methods, JsonRpcClient, JsonRpcMethodCallResult};

use methods::broadcast_tx_async::{
    RpcBroadcastTxAsyncRequest,
    CryptoHash,
    RpcBroadcastTxAsyncError,
};

let mainnet_client = JsonRpcClient::new().connect("https://rpc.mainnet.near.org");

let request = RpcBroadcastTxAsyncRequest {...}; // <-- create a valid request here

let result: errors::JsonRpcMethodCallResult<
    CryptoHash,
    RpcBroadcastTxAsyncError,
> = mainnet_client.call(&request);

• RPC Method Constructor: RpcBroadcastTxAsyncRequest
• RPC Method Response Type: CryptoHash
• RPC Method Error Type: RpcBroadcastTxAsyncError

[frol]

Macro magic gets out of hands 😄