doc: document zfs quota vs refquota
doc: document zfs quota vs refquota

Closes #2959.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,atxwebs/lxd,mjeanson/lxd,erikmack/lxd,georghopp/lxd,stgraber/lxd,stgraber/lxd,georghopp/lxd,mjeanson/lxd,mjeanson/lxd,hallyn/lxd,Skarlso/lxd,techtonik/lxd,atxwebs/lxd,lxc/lxd,stgraber/lxd,jsavikko/lxd,Skarlso/lxd,Skarlso/lxd,erikmack/lxd,pcdummy/lxd,lxc/lxd,pcdummy/lxd,jsavikko/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,georghopp/lxd,techtonik/lxd,hallyn/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,jsavikko/lxd,erikmack/lxd,atxwebs/lxd,atxwebs/lxd,techtonik/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,Skarlso/lxd,pcdummy/lxd