Update App Clips API documentation to include Backlinks.

- Backlinks is the contextual information that is returned via ClipData.
- Apps can provide this contextual data by integrating with
  AssistContent.
- When screenshot is captured, SystemUI queries the apps whose
  screenshot is taken for AssistContent and returns this via ClipData.
- Also updated the doc to mention checking
  EXTRA_CAPTURE_CONTENT_FOR_NOTE_STATUS_CODE, this way the Intent API
  constants are better connected.

Test: N/A
Flag: EXEMPT documentation update
Bug: 300307759
Change-Id: Ieb4eacbaa097da68374b235980537809623b38a1