Document preference value "KeepAVAudioSessionAlwaysActive" used in config.xml

# Feature Request

## Motivation Behind Feature
Saves many hours of debugging!

## Feature Description
There must be a preference value set in the config.xml file in order to be able to switch to another audio track when playing music in the background on iOS. This should be written in the documentation of the plugin.

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Start by reviewing the plugin documentation and the config.xml preference example mentioned in the issue. Document the iOS KeepAVAudioSessionAlwaysActive preference, its YES value, and that it enables switching audio tracks during background playback.

Written by the indexing model from the issue text.