Overlay network: magic behind VxLan interface?

Hi dear community,

Few days ago, i posted a question on stackoverflow.

I would like to know how the traffic on the overlay network could go to the outside world?

On this slideshare, i found a beginning of answer:

Vxlan must created in host namespace and moved to overlay namespace, otherwise vxlan could not interact with outside

Someone could point me to the real & official explanation (man pages, docker blog article etc.)?

Many thanks!
Greg