Changes to underlying filesystems
---------------------------------
-Offline changes, when the overlay is not mounted, are allowed to either
-the upper or the lower trees.
-
Changes to the underlying filesystems while part of a mounted overlay
filesystem are not allowed. If the underlying filesystem is changed,
the behavior of the overlay is undefined, though it will not result in
a crash or deadlock.
+Offline changes, when the overlay is not mounted, are allowed to the
+upper tree. Offline changes to the lower tree are only allowed if the
+"metadata only copy up", "inode index", and "redirect_dir" features
+have not been used. If the lower tree is modified and any of these
+features has been used, the behavior of the overlay is undefined,
+though it will not result in a crash or deadlock.
+
When the overlay NFS export feature is enabled, overlay filesystems
behavior on offline changes of the underlying lower layer is different
than the behavior when NFS export is disabled.