Update user manual

[nicobo]

The doc for the Android app (both as published at https://docs.nextcloud.com/android/android_app.html and inside this repository) is outdated : wrong version number, missing or moved menus, not enough details on existing features (how does the 'synchronize' menu works ? What's the use of the 'storage path' field, ...)

It's quite confusing for users...

[stale]

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