util: Document limit-related functions
util: Document limit-related functions

We're going to change their behavior, so it's good to have the
current one documented to serve as baseline.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

jfehlig/libvirt,jfehlig/libvirt,jfehlig/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,nertpinx/libvirt,olafhering/libvirt,crobinso/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,nertpinx/libvirt,nertpinx/libvirt