doc(bigtable): compiler warnings for ClientOptions

[dbolduc]

Part of the work for #7230

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This change is 

[google-cloud-cpp-bot]

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For commit: f53c866f24306ffb767bcc59ffd80672806c68e5

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[codecov]

Codecov Report

Merging #8914 (f53c866) into main (baa88cf) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #8914      +/-   ##
==========================================
- Coverage   93.24%   93.24%   -0.01%     
==========================================
  Files        1477     1477              
  Lines      125053   125055       +2     
==========================================
+ Hits       116610   116611       +1     
- Misses       8443     8444       +1     

Impacted Files	Coverage Δ
google/cloud/bigtable/client_options.h	100.00% <ø> (ø)
google/cloud/bigtable/client_options_test.cc	99.74% <ø> (ø)
google/cloud/bigtable/data_client.h	100.00% <ø> (ø)
google/cloud/bigtable/instance_admin_client.h	100.00% <ø> (ø)
...oogle/cloud/bigtable/instance_admin_client_test.cc	100.00% <ø> (ø)
google/cloud/bigtable/client_options.cc	100.00% <100.00%> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	97.75% <0.00%> (-0.50%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.72% <0.00%> (+0.56%)	⬆️

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