plugins/cilium-cni: cni-install - document CILIUM_CUSTOM_CNI_CONF purpose
plugins/cilium-cni: cni-install - document CILIUM_CUSTOM_CNI_CONF purpose

Signed-off-by: Timo Beckers <98c2d5a1e48c998bd9ba9dbc53d6857beae1c9bd@isovalent.com>

tklauser/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tgraf/cilium