fix(docs): Updated mutateExistingOnPolicyUpdate documentation to include validation info

[mvaal]

Related issue

Proposed Changes

Documentation around mutateExistingOnPolicyUpdate was not complete and I kept running into the same issue forgetting to include the mutate target. This add that additional information to the documentation.

Here is the corresponding kyverno validation code: https://github.com/kyverno/kyverno/blob/43173e6c76a6381ab27b8b2979c82d265c605ccf/api/kyverno/v1/spec_types.go#L360

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[mvaal]

Also, this documentation seems to be wrong:

When defining a list of targets[], the fields name and namespace are not strictly required but encouraged. If omitted, it implies a wildcard ("*") for the omitted field which can have unintended impact on other resources.

From my experience, namespace is required. I can make that fix here as well if confirmed.

[realshuting]

Thanks for the contribution @mvaal !

Also, this documentation seems to be wrong:

When defining a list of targets[], the fields name and namespace are not strictly required but encouraged. If omitted, it implies a wildcard ("*") for the omitted field which can have unintended impact on other resources.

From my experience, namespace is required. I can make that fix here as well if confirmed.

Why namespace is required for targets? A target could be a cluster-wide resource.

[mvaal]

Hey @realshuting!

Why namespace is required for targets? A target could be a cluster-wide resource.

That was my use case. I was running into some error where it kept saying that targets[*].namespace was required, but now I am unable to reproduce it, so you can ignore that comment. Sorry!

[realshuting]

Thanks for your contribution @mvaal !

[mvaal]

I fixed a consistency issue and updated the docs to be a little more readable based on conversation found here:
https://kubernetes.slack.com/archives/CLGR9BJU9/p1726606583673049

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