Document page.loading and page.loadingProgress

[wachunga]

As added in 1.9 here: #11091.

Currently undocumented:
https://github.com/ariya/phantomjs/wiki/API-Reference-WebPage

[stale]

Due to our very limited maintenance capacity, we need to prioritize our development focus on other tasks. Therefore, this issue will be automatically closed (see #15395 for more details). In the future, if we see the need to attend to this issue again, then it will be reopened. Thank you for your contribution!