doc: add not about escaping btrfs qgroups
doc: add not about escaping btrfs qgroups

Closes #3135.

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

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