doc: add mention of GOOGLE_CLOUD_CPP_TRACING_OPTIONS env var

[devbww]

This change is 

[codecov]

Codecov Report

Merging #1448 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1448      +/-   ##
==========================================
+ Coverage   95.56%   95.59%   +0.03%     
==========================================
  Files         191      190       -1     
  Lines       15794    15792       -2     
==========================================
+ Hits        15093    15096       +3     
+ Misses        701      696       -5

Impacted Files	Coverage Δ
google/cloud/spanner/internal/log_wrapper.h	71.42% <0%> (-13.19%)	⬇️
google/cloud/spanner/internal/session.h	100% <0%> (ø)	⬆️
google/cloud/spanner/internal/transaction_impl.h	100% <0%> (ø)	⬆️
google/cloud/spanner/instance_admin_connection.h
...anner/integration_tests/client_integration_test.cc	98.27% <0%> (ø)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.97% <0%> (ø)	⬆️
google/cloud/spanner/internal/spanner_stub.cc	78.49% <0%> (+1.07%)	⬆️
google/cloud/spanner/internal/polling_loop.h	92.85% <0%> (+9.52%)	⬆️

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