CDD: Clarify data deletion requirements

- Make it clear that all generated data, not just user-generated data
should be deleted on factory device reset.
- Clarify that only operating system files on read-only filesystems are
exempt from being deleted.

Bug: 124238463
Test: None
Change-Id: I3cd0bb57ed2c425763b7a50849dc216bc5dcab50
1 file changed