[feature/mdm-doc-upd] Update system activity examples in MDM documentation

[felix-schwarz]

Description

• include undocumented system activity identifier "com.apple.DocumentManagerUICore.SaveToFiles" in documentation as it is a common use-case
• expand description for "Save to camera roll" to include additional labels for that system activity ("Save Image", "Save Video")

Related Issue

https://github.com/owncloud/enterprise/issues/6891

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

[CLAassistant]

All committers have signed the CLA.