docs(iam): added comments on private_key_data

[Sita04]

This change is 

[google-cloud-cpp-bot]

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[coryan]

We do not leave TODO(Developer) marks in our examples. Also note that these examples are executable as-is, the parameters are received from the command-line, so the developer may not see this comment. I think instead we should do something like:

  std::cout << "ServiceAccountKey successfully created: "
                  << response->DebugString() << "\n";
                  <<  "Please save the key in a secure location, as they cannot be downloaded later\n";

[Sita04]

Thank you for the insight. Modified the comment.

[codecov]

Codecov Report

Merging #8204 (4c6ab44) into main (6ff74ae) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #8204      +/-   ##
==========================================
- Coverage   94.63%   94.62%   -0.01%     
==========================================
  Files        1323     1323              
  Lines      118583   118583              
==========================================
- Hits       112218   112213       -5     
- Misses       6365     6370       +5     

Impacted Files	Coverage Δ
google/cloud/storage/internal/curl_handle.h	80.00% <0.00%> (-2.86%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.75% <0.00%> (-1.25%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.75% <0.00%> (-0.36%)	⬇️
.../cloud/storage/benchmarks/throughput_experiment.cc	74.87% <0.00%> (+0.50%)	⬆️
...le/cloud/internal/default_completion_queue_impl.cc	97.72% <0.00%> (+0.56%)	⬆️

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[coryan]

FYI, the robots do not like the extra space at the end of one of your comments. Public versions of the logs are available from #8204 (comment)

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[coryan]

These instructions may help you speed up the edit -> test -> edit cycle:

https://github.com/googleapis/google-cloud-cpp/blob/main/doc/contributor/howto-guide-forks-and-pull-requests.md#reproducing-ci-build-failures

https://github.com/googleapis/google-cloud-cpp/blob/main/doc/contributor/howto-guide-running-ci-builds-locally.md

Please refrain from starting more builds until at least checkers passes locally on your workstation. We have limited capacity for Windows and macOS builds.

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[coryan]

Thanks for the PR! I will fix the build problem and merge this later today. We are in the middle of a release so please don't merge quite yet.