docs: domain: document blkiotune {read, write}_{bytes, iops}_sec
docs: domain: document blkiotune {read, write}_{bytes, iops}_sec

Added with commit 3b431929 in v1.2.2 but never documented

https://bugzilla.redhat.com/show_bug.cgi?id=1313613

fabianfreyer/libvirt,eskultety/libvirt,olafhering/libvirt,olafhering/libvirt,jfehlig/libvirt,libvirt/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,nertpinx/libvirt,nertpinx/libvirt,datto/libvirt,datto/libvirt,crobinso/libvirt,jfehlig/libvirt,datto/libvirt,taget/libvirt,fabianfreyer/libvirt,zippy2/libvirt,andreabolognani/libvirt,datto/libvirt,VenkatDatta/libvirt,eskultety/libvirt,datto/libvirt,libvirt/libvirt,fabianfreyer/libvirt,eskultety/libvirt,andreabolognani/libvirt,libvirt/libvirt,taget/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jardasgit/libvirt,eskultety/libvirt,jfehlig/libvirt,eskultety/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,olafhering/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,taget/libvirt,zippy2/libvirt,nertpinx/libvirt,zippy2/libvirt,taget/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,olafhering/libvirt,zippy2/libvirt,crobinso/libvirt,VenkatDatta/libvirt,crobinso/libvirt,andreabolognani/libvirt,crobinso/libvirt,jfehlig/libvirt,taget/libvirt