Update docs

test/proxy/beacon/UpgradeableBeacon.test.js

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+const { expectRevert, expectEvent } = require('@openzeppelin/test-helpers');
+const { expect } = require('chai');
+
+const UpgradeableBeacon = artifacts.require('UpgradeableBeacon');
+const Implementation1 = artifacts.require('Implementation1');
+const Implementation2 = artifacts.require('Implementation2');
+
+contract('UpgradeableBeacon', function (accounts) {
+  const [owner, other] = accounts;
+
+  it('cannot be created with non-contract implementation', async function () {
+    await expectRevert(
+      UpgradeableBeacon.new(accounts[0]),
+      'UpgradeableBeacon: implementation is not a contract',
+    );
+  });
+
+  context('once deployed', async function () {
+    beforeEach('deploying beacon', async function () {
+      this.v1 = await Implementation1.new();
+      this.beacon = await UpgradeableBeacon.new(this.v1.address, { from: owner });
+    });
+
+    it('returns implementation', async function () {
+      expect(await this.beacon.implementation()).to.equal(this.v1.address);
+    });
+
+    it('can be upgraded by the owner', async function () {
+      const v2 = await Implementation2.new();
+      const receipt = await this.beacon.upgradeTo(v2.address, { from: owner });
+      expectEvent(receipt, 'Upgraded', { implementation: v2.address });
+      expect(await this.beacon.implementation()).to.equal(v2.address);
+    });
+
+    it('cannot be upgraded to a non-contract', async function () {
+      await expectRevert(
+        this.beacon.upgradeTo(other, { from: owner }),
+        'UpgradeableBeacon: implementation is not a contract',
+      );
+    });
+
+    it('cannot be upgraded by other account', async function () {
+      const v2 = await Implementation2.new();
+      await expectRevert(
+        this.beacon.upgradeTo(v2.address, { from: other }),
+        'Ownable: caller is not the owner',
+      );
+    });
+  });
+});