Add option --enable-media-hub-audio to "Command line arguments" section

[Peter Bittner]

Web page https://developer.ubuntu.com/en/web/ubuntu-webapps-guide/ lists a set of command line options for `webapp-container` in the "Command line arguments" section (4/5th down the page). A note says:

"Note: Only the following options apply to converged Ubuntu"

However, the list of is not complete. it should (at least) include the recently added "--enable-media-hub-audio" option:

* `--enable-media-hub-audio` Set environment variable OXIDE_ENABLE_MEDIA_HUB_AUDIO=1 to allow playing audio when web app is in background.

Notes
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Other relevant options are not known to the reporter of this bug. It may be worth following up with the development team in charge of `webapp-container`.

Other Details
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Discussion on mailing list:
https://lists.launchpad.net/ubuntu-phone/msg18649.html

[David Callé (davidc3)]

Thanks Peter, last time I tried, it wasn't fully working so it was kept out of the guide. I get from the thread that it is fixed, there will be an update soon.

[Peter Bittner (peter-bittner)]

Please don't forget to update the online developer resources.