doc: exclude benchmarks for doxygen docs

[coryan]

The classes in the benchmarks are not interesting for the user-facing
reference documentation.

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This change is 

[codecov]

Codecov Report

Merging #1208 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1208      +/-   ##
==========================================
- Coverage   95.66%   95.63%   -0.04%     
==========================================
  Files         180      179       -1     
  Lines       13635    13648      +13     
==========================================
+ Hits        13044    13052       +8     
- Misses        591      596       +5

Impacted Files	Coverage Δ
...loud/spanner/internal/partial_result_set_resume.cc	90.9% <0%> (-4.55%)	⬇️
google/cloud/spanner/internal/retry_loop.h	95.45% <0%> (-4.55%)	⬇️
google/cloud/spanner/internal/polling_loop.h	91.89% <0%> (-2.71%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	85.71% <0%> (-2.07%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	81.57% <0%> (-1.76%)	⬇️
...loud/spanner/internal/partial_result_set_source.cc	93.24% <0%> (-1.36%)	⬇️
google/cloud/spanner/keys.h	100% <0%> (ø)	⬆️
google/cloud/spanner/internal/transaction_impl.h	100% <0%> (ø)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	92.37% <0%> (ø)	⬆️
google/cloud/spanner/transaction.h	100% <0%> (ø)	⬆️
... and 3 more

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