[iOS] Update documentation for iOS app version 1.1.1

[hosy]

The new iOS app got some new features in version 1.1.0, which should be documented.

What Needs to be Documented?

System Dark Mode Support

owncloud/ios-app#489

What Type Of Content Change Is This? (Optional)

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

Which Manual Does This Relate To? (Optional)

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other

[hosy]

@michaelstingl in cc for you

[settermjd]

@hosy, please add some screenshots for the documentation of the system dark mode (owncloud/ios-app#489).