Fix docs in the extending-contracts section (#5300)

contracts/mocks/docs/access-control/AccessControlNonRevokableAdmin.sol

@@ -0,0 +1,17 @@
+// contracts/AccessControlNonRevokableAdmin.sol
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+import {AccessControl} from "../../../access/AccessControl.sol";
+
+contract AccessControlNonRevokableAdmin is AccessControl {
+    error AccessControlNonRevokable();
+
+    function revokeRole(bytes32 role, address account) public override {
+        if (role == DEFAULT_ADMIN_ROLE) {
+            revert AccessControlNonRevokable();
+        }
+
+        super.revokeRole(role, account);
+    }
+}