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 <[email protected]>

Author: rata

keps/sig-node/0753-sidecarcontainers.md

@@ -553,8 +553,7 @@ them][kinvolk-poc-sidecar-test-commit] for instruction on how to run it.
 Some other things worth noting about the implementation:
  * It is done using pod annotations so it is easy to test for users (doesn't
    modify pod spec)
- * It implements the KEP proposal and not the suggested modifications, with one
-   exception: the podPhase is not modified.
+ * Wasn't updated to call preStop hooks one time only, as this KEP now proposes
  * There is some c&p code to avoid doing refactors and just have a patch that is
    simpler to cherry-pick on different Kubernetes versions.