rootfs-handoff: Milestone D - adapt /sbin/init and fstab for post-handoff

rootfs-handoff: Milestone D - adapt /sbin/init and fstab for post-handoff

- Updated /etc/fstab with documentation for post-handoff configuration
- Documented that root filesystem is already mounted by kernel on /newroot
- Documented that early /init performs pivot_root to make /newroot the new root
- Documented that virtual filesystems are moved by early /init
- Documented that /sbin/init runs after handoff and skips already-mounted filesystems
- /sbin/init already has is_mounted() check to avoid duplicate mounts
- No code changes needed in /sbin/init - already compatible with handoff

Test Results:
- Smoke test: 131/131 PASS
- Battery test: 157/157 PASS
- Analyzer: PASS
- Zero regressions