Update mobile.md

Added note stating that notebooks and notes are not backed up when performing a backup to a mac. It's important not to reply on iphone backups to backup note content.

readme/apps/mobile.md

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 <img src="https://raw.githubusercontent.com/laurent22/joplin/dev/Assets/WebsiteAssets/images/DemoMobile.jpg" style="width: 100%" />
 
 An Android and iOS (iPhone/iPad) applications are available from the [Joplin home page](https://joplinapp.org).
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+Notebooks and notes on iOS are not backed up when [backing up to your Mac](https://support.apple.com/guide/mac-help/back-up-and-restore-your-device-mchla3c8ed03/mac).
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