PXE docs need more info on persistent volumes

[pgburt]

Taken from this google group discussion.

Problem

User can't find good documentation about persistent volumes for PXE.

Details

I linked the user to this doc:
https://coreos.com/os/docs/latest/booting-with-pxe.html

Part of their reply:

Only partly. This is actually the documentation that started my
confusion, as it gives so many options without really explaining them,
and does not give advice like "this ist the best practice for this
environment". See, for example, the etc web page that explains three
or four ways of building differently sized etcd clusters (which has
the disadvantage that I fail to find it each and every time I want to
look at it).

They would like explicit instructions re: persistent volumes.

Taken from this google group discussion.