Update docs

contracts/mocks/SignedSafeMathMock.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../utils/math/SignedSafeMath.sol";
+
+contract SignedSafeMathMock {
+    function mul(int256 a, int256 b) public pure returns (int256) {
+        return SignedSafeMath.mul(a, b);
+    }
+
+    function div(int256 a, int256 b) public pure returns (int256) {
+        return SignedSafeMath.div(a, b);
+    }
+
+    function sub(int256 a, int256 b) public pure returns (int256) {
+        return SignedSafeMath.sub(a, b);
+    }
+
+    function add(int256 a, int256 b) public pure returns (int256) {
+        return SignedSafeMath.add(a, b);
+    }
+}