Update docs

test/security/ReentrancyGuard.test.js

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+const { expectRevert } = require('@openzeppelin/test-helpers');
+
+const { expect } = require('chai');
+
+const ReentrancyMock = artifacts.require('ReentrancyMock');
+const ReentrancyAttack = artifacts.require('ReentrancyAttack');
+
+contract('ReentrancyGuard', function (accounts) {
+  beforeEach(async function () {
+    this.reentrancyMock = await ReentrancyMock.new();
+    expect(await this.reentrancyMock.counter()).to.be.bignumber.equal('0');
+  });
+
+  it('nonReentrant function can be called', async function () {
+    expect(await this.reentrancyMock.counter()).to.be.bignumber.equal('0');
+    await this.reentrancyMock.callback();
+    expect(await this.reentrancyMock.counter()).to.be.bignumber.equal('1');
+  });
+
+  it('does not allow remote callback', async function () {
+    const attacker = await ReentrancyAttack.new();
+    await expectRevert(
+      this.reentrancyMock.countAndCall(attacker.address), 'ReentrancyAttack: failed call');
+  });
+
+  // The following are more side-effects than intended behavior:
+  // I put them here as documentation, and to monitor any changes
+  // in the side-effects.
+  it('does not allow local recursion', async function () {
+    await expectRevert(
+      this.reentrancyMock.countLocalRecursive(10), 'ReentrancyGuard: reentrant call',
+    );
+  });
+
+  it('does not allow indirect local recursion', async function () {
+    await expectRevert(
+      this.reentrancyMock.countThisRecursive(10), 'ReentrancyMock: failed call',
+    );
+  });
+});