lazy-pages: add comments to update_lazy_iovecs

This function does weird things, so better have it at least somehow
documented.

travis-ci: success for lazy-pages: add comments to update_lazy_iovecs
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>