Apple: Document Data Collected

[philprime]

Description

See getsentry/team-sdks#82

[kahest]

WIP on Java/Android via getsentry/sentry-java#4092 - we can take a few hints based on the Android docs

[kahest]

Re-opening this because we need to add information on which of the collected data items are guarded by sendDefaultPii. We're also missing collected user information (most importantly ip address). See the Android docs as example