doc(spanner): add an example for CommitAtLeastOnce()

[devbww]

This change is 

[codecov]

Codecov Report

Patch coverage: 68.75% and project coverage change: -0.01 ⚠️

Comparison is base (dcc823e) 93.66% compared to head (87cdc8f) 93.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11905      +/-   ##
==========================================
- Coverage   93.66%   93.66%   -0.01%     
==========================================
  Files        1838     1838              
  Lines      166473   166489      +16     
==========================================
+ Hits       155931   155939       +8     
- Misses      10542    10550       +8     

Impacted Files	Coverage Δ
google/cloud/spanner/client.h	100.00% <ø> (ø)
google/cloud/spanner/samples/samples.cc	61.87% <68.75%> (+0.04%)	⬆️

... and 7 files with indirect coverage changes

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