Add iOS documentation on batch processing level and background tasks

[maciejburda]

What does this PR do? What is the motivation?

Adds documentation on newly added public properties on the iOS RUM SDK.

Merge instructions

• [x] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/maciey/batch-processing-and-background-tasks-ios-rum/real_user_monitoring/mobile_and_tv_monitoring/advanced_configuration/ios