ovl: document copying layers restrictions with inodes index

The inodes index feature introduces a behavior change - on mount,
upper root origin file handle is verified to match the lower root dir.
This implies that copied layers cannot be mounted with the inodes index
feature enabled, without explicitly removing the upper dir origin xattr
and the index dir.

The inodes index feature is required to support:
- Prevent breaking hardlinks on copy up
- NFS export support (upcoming)
- Overlayfs snapshots (POC)

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>