sidecar: Update PoC implementation note
sidecar: Update PoC implementation note

After the last changes to the KEP, this doesn't apply anymore: the
podPhase is not modified.

Furthermore, the preStop hooks are sent once but the PoC wasn't updated
to reflect that. To avoid confussions, call it out there.

Signed-off-by: Rodrigo Campos <6dfa9cecb562e345739f2e4eb69e9ebd0fbff687@kinvolk.io>

kubernetes/enhancements,kubernetes/enhancements,kubernetes/enhancements