docs: add liquidator reference

Author: kkirka

core-pool/admins.md

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+# Admins and ownership
+
+Most of our contracts have a dedicated admin. The admin of most of the contracts is Governance. The admin can be changed with a two-step procedure that guarantees there's no error during transferring the rights.
+
+The interface is similar but for historical reasons some of the contracts do not revert in case of failure. These contracts return 0 if the transaction was successful and an error code otherwise.
+
+## _setPendingAdmin
+
+```
+function _setPendingAdmin(
+    address newPendingAdmin
+) external [returns (uint256 error)];
+```
+
+Begins transfer of admin rights. The newPendingAdmin must call `_acceptAdmin` to finalize the transfer. The function is only callable by admin.
+
+## _acceptAdmin
+
+```solidity
+function _acceptAdmin() external [returns (uint256)];
+```
+
+Accepts transfer of admin rights. The function is only callable by pendingAdmin.

core-pool/liquidator.md

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+# Liquidator
+
+Liquidator contract is the only publicly accessible interface for liquidations in the core pool.
+
+Liquidator contract was introduced in [VIP-64](https://app.venus.io/governance/proposal/64) so that the protocol could capture a part of the liquidation revenue. It has a unified interface for VAI, BNB and BEP-20 token liquidations.
+
+## liquidateBorrow
+
+```
+function liquidateBorrow(
+    address vToken,
+    address borrower,
+    uint256 repayAmount,
+    VToken vTokenCollateral
+) external payable;
+```
+
+Liquidates a borrow and splits the seized amount between treasury and liquidator. The liquidators should use this interface instead of calling `vToken.liquidateBorrow(...)` directly. Reverts on errors.
+
+Note: For BNB borrows `msg.value` must be equal to repayAmount; for all other borrows msg.value must be zero.
+
+To use this function, the liquidator has to `approve` the underlying asset to Liquidator (unless the underlying asset is BNB).
+
+#### Parameters
+
+| Name | Type | Description |
+| ---- | ---- | ----------- |
+| `vToken` | address | Borrowed vToken (or VAIUnitroller for VAI) |
+| `borrower` | address | The address of the borrower |
+| `repayAmount` | uint256 | The amount to repay on behalf of the borrower |
+| `vTokenCollateral` | address | The collateral vToken to seize |
+
+## setTreasuryPercent
+
+```
+function setTreasuryPercent(
+    uint256 newTreasuryPercentMantissa
+) external onlyAdmin;
+```
+
+Sets the new percent of the seized amount that goes to treasury. Should be less than or equal to `comptroller.liquidationIncentiveMantissa() - 1e18`.
+
+Only callable by the admin (Governance).
+
+#### Parameters
+
+| Name | Type | Description |
+| ---- | ---- | ----------- |
+| `newTreasuryPercentMantissa` | uint256 | New treasury percent (scaled by 10^18). |
+
+## _setPendingAdmin, _acceptAdmin
+
+The admin is updated via the regular [two-step procedure](./admins.md). `_setPendingAdmin` and `_acceptAdmin` do not return any value and revert on errors.