Update Page Objects doc for testing with node 8 async/await

[vsravuri]

Bug report

• Node Version: v8.9.3
• Protractor Version: 5.2.2

The latest Node.js provides native async/await. Control flow will be completely removed in future version of Selenium [Selenium 4.0]. The Page Object example provided in the http://www.protractortest.org/#/page-objects document is written using Control Flow.

I've referred to the http://www.protractortest.org/#/async-await document to understand how to convert a spec file written in Control Flow to async/await but struggling to find an example to convert the Page Objects written in Control flow to async/await.

Requirement:
Update the existing Page Object document to include an example to write Page Object using Node async/await. The examples in existing Page Object document will be obsolete after Control Flow is removed from Selenium.

[qiyigg]

This was done by #4675.
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