doc: update doc/userns-idmap.md for multiple idmaps
doc: update doc/userns-idmap.md for multiple idmaps

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>

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