ovl: update documentation w.r.t. constant inode numbers

This is most obvious from the 'st_dev' field returned by stat(2).

While directories will report an st_dev from the overlay-filesystem,

all non-directory objects will report an st_dev from the lower or

non-directory objects may report an st_dev from the lower filesystem or

upper filesystem that is providing the object.  Similarly st_ino will

only be unique when combined with st_dev, and both of these can change

over the lifetime of a non-directory object.  Many applications and

tools ignore these values and will not be affected.

In the special case of all overlay layers on the same underlying

filesystem, all objects will report an st_dev from the overlay

filesystem and st_ino from the underlying filesystem.  This will

make the overlay mount more compliant with filesystem scanners and

overlay objects will be distinguishable from the corresponding

objects in the original filesystem.

Upper and Lower

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