lib: Document that virConnectGetAllDomainStats may omit some stats fields
lib: Document that virConnectGetAllDomainStats may omit some stats fields

Add a note to make the users aware that some stats groups or fields may
be missing in certain cases.

elmarco/libvirt,datto/libvirt,crobinso/libvirt,jardasgit/libvirt,agx/libvirt,jfehlig/libvirt,shugaoye/libvirt,jfehlig/libvirt,shugaoye/libvirt,libvirt/libvirt,andreabolognani/libvirt,zippy2/libvirt,agx/libvirt,shugaoye/libvirt,olafhering/libvirt,libvirt/libvirt,fabianfreyer/libvirt,eskultety/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,elmarco/libvirt,nertpinx/libvirt,taget/libvirt,taget/libvirt,rlaager/libvirt,jfehlig/libvirt,olafhering/libvirt,taget/libvirt,datto/libvirt,crobinso/libvirt,andreabolognani/libvirt,nertpinx/libvirt,jardasgit/libvirt,elmarco/libvirt,olafhering/libvirt,andreabolognani/libvirt,crobinso/libvirt,shugaoye/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,libvirt/libvirt,nertpinx/libvirt,rlaager/libvirt,taget/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,olafhering/libvirt,eskultety/libvirt,agx/libvirt,agx/libvirt,datto/libvirt,libvirt/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,eskultety/libvirt,agx/libvirt,jardasgit/libvirt,rlaager/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,eskultety/libvirt,crobinso/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,datto/libvirt,zippy2/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,zippy2/libvirt,zippy2/libvirt,elmarco/libvirt,taget/libvirt,rlaager/libvirt,rlaager/libvirt,shugaoye/libvirt,datto/libvirt,eskultety/libvirt,elmarco/libvirt,fabianfreyer/libvirt