Update docs

test/utils/Counters.test.js

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+const { expectRevert } = require('@openzeppelin/test-helpers');
+
+const { expect } = require('chai');
+
+const CountersImpl = artifacts.require('CountersImpl');
+
+contract('Counters', function (accounts) {
+  beforeEach(async function () {
+    this.counter = await CountersImpl.new();
+  });
+
+  it('starts at zero', async function () {
+    expect(await this.counter.current()).to.be.bignumber.equal('0');
+  });
+
+  describe('increment', function () {
+    context('starting from 0', function () {
+      it('increments the current value by one', async function () {
+        await this.counter.increment();
+        expect(await this.counter.current()).to.be.bignumber.equal('1');
+      });
+
+      it('can be called multiple times', async function () {
+        await this.counter.increment();
+        await this.counter.increment();
+        await this.counter.increment();
+
+        expect(await this.counter.current()).to.be.bignumber.equal('3');
+      });
+    });
+  });
+
+  describe('decrement', function () {
+    beforeEach(async function () {
+      await this.counter.increment();
+      expect(await this.counter.current()).to.be.bignumber.equal('1');
+    });
+    context('starting from 1', function () {
+      it('decrements the current value by one', async function () {
+        await this.counter.decrement();
+        expect(await this.counter.current()).to.be.bignumber.equal('0');
+      });
+
+      it('reverts if the current value is 0', async function () {
+        await this.counter.decrement();
+        await expectRevert(this.counter.decrement(), 'Counter: decrement overflow');
+      });
+    });
+    context('after incremented to 3', function () {
+      it('can be called multiple times', async function () {
+        await this.counter.increment();
+        await this.counter.increment();
+
+        expect(await this.counter.current()).to.be.bignumber.equal('3');
+
+        await this.counter.decrement();
+        await this.counter.decrement();
+        await this.counter.decrement();
+
+        expect(await this.counter.current()).to.be.bignumber.equal('0');
+      });
+    });
+  });
+
+  describe('reset', function () {
+    context('null counter', function () {
+      it('does not throw', async function () {
+        await this.counter.reset();
+        expect(await this.counter.current()).to.be.bignumber.equal('0');
+      });
+    });
+
+    context('non null counter', function () {
+      beforeEach(async function () {
+        await this.counter.increment();
+        expect(await this.counter.current()).to.be.bignumber.equal('1');
+      });
+      it('reset to 0', async function () {
+        await this.counter.reset();
+        expect(await this.counter.current()).to.be.bignumber.equal('0');
+      });
+    });
+  });
+});