As of right now (March 2020), I'm not aware of any clear documentation, and have heard from others that there is not. I seem to recall that one of the goals of Trinity was to serve as an easier-to-parse implementation. In any event Py-EVM, their EVM implementation, should be more up-to-date than pyethereum. If you're specifically interested in the EVM, I ...
I'd like to get pending transactions with using geth via web3.js (or with using other client via other node.js library).
In order to get pending transactions using web3.js, the required steps are:
npm i web3
Create a file getPendingTx.js
const Web3 = require('web3');
const web3 = new Web3(new Web3.providers.HttpProvider('http://localhost:8545'));
To get the pending transactions you need a node that you're running on your own because this type of action requires a lot of resources.
You can run your own Geth node and wait until it is synchronized. When I want to run a Geth node on the main net and query for the transaction pool, I usually start it like this.
Download the Geth binary and rename it as ...