Update docs

contracts/mocks/ConditionalEscrowMock.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+import "../payment/escrow/ConditionalEscrow.sol";
+
+// mock class using ConditionalEscrow
+contract ConditionalEscrowMock is ConditionalEscrow {
+    mapping(address => bool) private _allowed;
+
+    function setAllowed(address payee, bool allowed) public {
+        _allowed[payee] = allowed;
+    }
+
+    function withdrawalAllowed(address payee) public view override returns (bool) {
+        return _allowed[payee];
+    }
+}