docs: add TODO for VFS umount root check (Item 11)

docs: add TODO for VFS umount root check (Item 11)

Check of root directory in umount is pending due to stability issues.
Two attempts failed:
- char root[128] caused memory corruption (alignment issues)
- fs_node_t* root_fs_node caused massive instability (55/125 tests)

Check of cwd is fully implemented and validated (124/124 tests passing).
Documented future implementation options:
- Per-process mount namespaces (most robust, 5-7 days)
- Fix alignment issues (medium, 2-3 days)
- Debug fs_node_t* approach (high, 3-4 days)

Recommendation: Keep cwd check only, implement root check when
chroot/pivot_root is actually needed or when implementing containers.