Add docs about configuring faults and retry/timeout on the same VirtualService

[denniseffing]

This PR adds documentation about a common issue when testing resilience mechanisms like retries and timeouts using the Istio fault injection: The resilience mechanisms don't work when faults and retrys/timeouts are configured on the same VirtualService. See istio/istio#13705 for more details.

I'm currently ironing out the details for signing the CLA as a codecentric employee, I expect that to be done sometime next week but I wanted to open this PR to collect some feedback.

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[istio-testing]

Hi @denniseffing. Thanks for your PR.

I'm waiting for a istio 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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[zirain]

@denniseffing please sign CLA, thx!

[Arhell]

/ok-to-test

[denniseffing]

/retest

[frankbu]

Not sure, but it might be worth mentioning the alternative of moving the retry policy to the virtual service of a client service, if that makes sense? For example, in bookinfo, it would probably be reasonable to put the retry in the reviews service, for fault injection in the ratings service.

[denniseffing]

That is true. I didn't mention this specifically because configuring retries and faults on different virtual services is quite the obvious workaround but not always applicable. I'll see if I can come up with a sentence or two that mentions this as well.

[frankbu]

Overall LGTM. Just a couple of minor suggestions.

[ericvn]

@denniseffing Still waiting on you satisfying the CLA bot.

[denniseffing]

Yes, sorry! I made the "mistake" of asking internally which one we would like to sign. 😅 I honestly didn't expect that much delay, my apologies. I will sign them soon!

[denniseffing]

@ericvn I signed the CLA and rebased the changes.