ovl: update doc

moves it over to the old name.  The new file may be on a different

filesystem, so both st_dev and st_ino of the file may change.

Any open files referring to this inode will access the old data and

metadata.  Similarly any file locks obtained before copy_up will not

apply to the copied up file.

Any open files referring to this inode will access the old data.

On a file opened with O_RDONLY fchmod(2), fchown(2), futimesat(2) and

fsetxattr(2) will fail with EROFS.

Any file locks (and leases) obtained before copy_up will not apply

to the copied up file.

If a file with multiple hard links is copied up, then this will

"break" the link.  Changes will not be propagated to other names