WithdrawalVault contract doc

Author: folkyatina

docs/contracts/withdrawal-vault.md

@@ -3,9 +3,78 @@
 - [Source Code](https://github.com/lidofinance/lido-dao/blob/master/contracts/0.8.9/WithdrawalVault.sol)
 - [Deployed Contract](https://etherscan.io/address/0xb9d7934878b5fb9610b3fe8a5e441e8fad7e293f)
 
-A vault that corresponds to the type-0x01 Lido withdrawal credentials.
+## What is WithdrawalVault
+
+A simple contract that accumulates partial and full withdrawals that comes from Beacon chain. Its address corresponds to the type-0x01 Lido withdrawal credentials.
+During the accounting oracle report the vault is emptied by Lido to to the internal buffer, see [here](lido.md#oracle-report) for details.
+
+Vault is recoverable, so any tokens or ether can be trasferred to the treasury by DAO.
+
 The currently deployed version is upgradable because of anticipated changes in Ethereum withdrawals mechanics.
 
 :::note
 The contract is meant to be ossified somewhere after withdrawal credentials triggerable exits are implemented.
 :::
+
+## View methods
+
+### getContractVersion()
+
+Returns the current contract version.
+
+```sol
+function getContractVersion() returns (uint256)
+```
+
+## Methods
+
+### withdrawWithdrawals()
+
+Transfer `_amount` of accumulated withdrawals to the Lido contract.
+
+:::note
+
+Can be called only by the [Lido](lido.md) contract
+
+:::
+
+```sol
+function withdrawWithdrawals(uint256 _amount)
+```
+
+### recoverERC20()
+
+Transfers the given amount of the ERC20-token (defined by the provided token contract address)
+currently belonging to the vault contract address to the Lido treasury address.
+
+Emits the `ERC20Recovered` event.
+
+
+```sol
+function recoverERC20(address _token, uint256 _amount) external
+```
+
+#### Parameters:
+
+| Name       | Type      | Description             |
+| ---------- | --------- | ----------------------- |
+| `_token`   | `address` | ERC20-compatible token  |
+| `_amount`  | `uint256` | token amount to recover |
+
+### recoverERC721()
+
+Transfers the given tokenId of the ERC721-compatible NFT (defined by the provided token contract address)
+currently belonging to the vault contract address to the Lido treasury address.
+
+Emits the `ERC721Recovered` event.
+
+```sol
+function recoverERC721(address _token, uint256 _tokenId) external
+```
+
+#### Parameters:
+
+| Name       | Type      | Description             |
+| ---------- | --------- | ----------------------- |
+| `_token`   | `address` | ERC721-compatible token |
+| `_tokenId` | `uint256` | minted token id         |