Add notes on timeouts while using long GRPC streams

[roemba]

What this PR does / why we need it:
GRPC streams longer than 60s hit multiple timeouts that NGINX has defined. Not all of them are as clear for a user to find (do not show clearly in NGINX debug logs), so I added some notes to the GRPC example to warn users of setting the correct timeouts if they wish their stream to not be aborted after 60 seconds.

Which issue this PR fixes:
No specific issue, more fixing of missing information in the documentation.

Special notes for your reviewer:

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[k8s-ci-robot]

Hi @roemba. Thanks for your PR.

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[aledbf]

/ok-to-test

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[aledbf]

/lgtm
/approve

[aledbf]

@roemba thanks!

[roemba]

/assign @aledbf

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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