docs: fix CNI setup mistakes
docs: fix CNI setup mistakes

The documentation erroneously stated that CNI configuration
would be preserved. I verified that this isn't true; the CNI file is
always overwritten (unless CNI management is disabled).

Also, an environment variable was confuslingly described.

Signed-off-by: Casey Callendrello <11fb6d5a105a66c85408b8005c461d53818b736e@isovalent.com>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium