doc: added hint why regular expressions might not be accepted

[Herr-Sepp]

Kubernetes validates all regular expressions using RE2 which does not support the full syntax of PCRE which uses NGINX.

see: #4989

What this PR does / why we need it:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Which issue/s this PR fixes

How Has This Been Tested?

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

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[k8s-ci-robot]

Hi @Herr-Sepp. 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.

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[aledbf]

/check-cla

[aledbf]

/ok-to-test

[aledbf]

/lgtm
/approve

[aledbf]

@Herr-Sepp thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, Herr-Sepp

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1119281). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #4991   +/-   ##
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  Coverage          ?   58.59%           
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  Files             ?       88           
  Lines             ?     6765           
  Branches          ?        0           
=========================================
  Hits              ?     3964           
  Misses            ?     2368           
  Partials          ?      433

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[k8s-ci-robot]

@Herr-Sepp: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-ingress-nginx-e2e-1-15	3f4da0f	link	/test pull-ingress-nginx-e2e-1-15

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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. I understand the commands that are listed here.