doc(storage): document per-operation options

[coryan]

Fixes #7691

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This change is 

[google-cloud-cpp-bot]

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Codecov Report

Merging #9247 (629f657) into main (5d88584) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #9247   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files        1457     1458    +1     
  Lines      131845   131934   +89     
=======================================
+ Hits       124690   124779   +89     
  Misses       7155     7155           

Impacted Files	Coverage Δ
google/cloud/storage/client.h	99.83% <ø> (ø)
...e/examples/storage_client_per_operation_samples.cc	100.00% <100.00%> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	97.75% <0.00%> (-0.50%)	⬇️
...loud/bigtable/internal/connection_refresh_state.cc	100.00% <0.00%> (+2.77%)	⬆️

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[devbww]

Is this true? If so, I'm not sure it is a feature.

[coryan]

We can have a discussion about whether we should change this, but this is not new behavior. Note that this is referring to the static options, so something like this would fail to compile:

  client.GetBucketMetadata("my-bucket",
     gcs::Generation(7) // compile-time error, buckets do not have generations
 );

while this would compile (and this is new behavior):

  client.GetBucketMetadata("my-bucket",
    g::Options{}.set<pubsub::RetryPolicyOption(...) // ignored
  );

[devbww]

That's cool. I was only thinking about the behavior for Options.

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