doc: add more snippets to documentation

[coryan]

This change is 

[codecov]

Codecov Report

Merging #1331 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1331      +/-   ##
==========================================
+ Coverage    94.6%   94.66%   +0.05%     
==========================================
  Files         172      172              
  Lines       13623    13616       -7     
==========================================
+ Hits        12888    12889       +1     
+ Misses        735      727       -8

Impacted Files	Coverage Δ
google/cloud/spanner/row.h	90% <ø> (ø)	⬆️
google/cloud/spanner/mutations.h	100% <ø> (ø)	⬆️
google/cloud/spanner/samples/samples.cc	89.34% <100%> (-0.18%)	⬇️
google/cloud/spanner/internal/log_wrapper.h	71.42% <0%> (-3.58%)	⬇️
google/cloud/spanner/internal/transaction_impl.h	100% <0%> (ø)	⬆️
...cloud/spanner/internal/partial_result_set_resume.h	100% <0%> (ø)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.97% <0%> (ø)	⬆️
google/cloud/spanner/database_admin_connection.h
google/cloud/spanner/instance_admin_connection.h	100% <0%> (ø)
... and 7 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update df53bc1...c0a1b0e. Read the comment docs.