📖 Document pitfalls of CreateOrUpdate and CreateOrPatch

[ian-howell]

This updates the documentation for the CreateOrUpdate and CreateOrPatch methods to include a section on the pitfalls of using these methods.

Closes #3191

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[k8s-ci-robot]

Hi @ian-howell. Thanks for your PR.

I'm waiting for a kubernetes-sigs 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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[troy0820]

/ok-to-test

I want to make sure the godoc renders like we want it to and validate the additions to the documentation as well.

/assign @alvaroaleman
/hold

[ian-howell]

I had some free time this morning, so I went ahead and wrote up some unit tests which validate the claims in this PR.

Here's the branch: https://github.com/ian-howell/controller-runtime/tree/doc-update-verification. It includes a change to the test-all.sh script that causes make test to only run the relevant tests.

Here's the commit for convenience: ian-howell@1ac6810