[docs] Add info about x-forwarded-prefix breaking change

[Miouge1]

What this PR does / why we need it:
#3786 changed the way the x-forwarded-prefix annotation worked. Informing users in release notes helps with to take appropriate action when upgrading.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

I am using the x-forwarded-prefix: true annotation and ran into problems when upgrading from v0.21 to v0.26.

I am not sure about the appropriate mitigation, so far the following worked for me:

v0.21

apiVersion: networking.k8s.io/v1beta1
kind: Ingress
metadata:
  name: test-ingress
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /
    nginx.ingress.kubernetes.io/x-forwarded-prefix: "true"
spec:
  rules:
  - http:
      paths:
      - path: /testpath/
        backend:
          serviceName: test
          servicePort: 80

v0.26

apiVersion: networking.k8s.io/v1beta1
kind: Ingress
metadata:
  name: test-ingress
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /
    nginx.ingress.kubernetes.io/x-forwarded-prefix: "/testpath/"
spec:
  rules:
  - http:
      paths:
      - path: /testpath/
        backend:
          serviceName: test
          servicePort: 80

[k8s-ci-robot]

Hi @Miouge1. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[aledbf]

/ok-to-test

[aledbf]

/lgtm

[aledbf]

@Miouge1 thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, Miouge1

The full list of commands accepted by this bot can be found here.

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[codecov-io]

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@@            Coverage Diff            @@
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  Branches          ?        0           
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  Misses            ?     2355           
  Partials          ?      437

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