docs: add snippets for methods and classes in keys.h

[scotthart]

This change is 

[coryan]

See below, but if you feel I am wrong just ping me over IM.

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @devbww, @devjgm, @mr-salty, and @scotthart)

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google/cloud/spanner/samples/samples.cc, line 1055 at r1 (raw file):

  auto delete_singers =
      spanner::DeleteMutationBuilder(
          "Singers", spanner::KeySet().AddRange(spanner::MakeKeyBoundOpen(0),

I was wondering about consistency with other languages (this is a sample in the cloud site docs), but they are hilariously inconsistent.

https://cloud.google.com/spanner/docs/modify-mutation-api#deleting_rows_in_a_table

Nevertheless, C++ would be the only language using an open range to delete the rows, that seems odd.

Probably best to create another example.

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google/cloud/spanner/samples/samples.cc, line 2435 at r1 (raw file):

  namespace spanner = ::google::cloud::spanner;
  RemoteConnectionFake remote_connection;
  //! [key-set-builder]

nit: I think these tags could go in the same line too, as above?

[scotthart]

Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on @coryan, @devbww, @devjgm, and @mr-salty)

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google/cloud/spanner/samples/samples.cc, line 1055 at r1 (raw file):

Previously, coryan (Carlos O'Ryan) wrote…

I was wondering about consistency with other languages (this is a sample in the cloud site docs), but they are hilariously inconsistent.

https://cloud.google.com/spanner/docs/modify-mutation-api#deleting_rows_in_a_table

Nevertheless, C++ would be the only language using an open range to delete the rows, that seems odd.

Probably best to create another example.

Done.

[coryan]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @devbww, @devjgm, and @mr-salty)

[codecov]

Codecov Report

Merging #1451 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1451      +/-   ##
==========================================
+ Coverage   95.56%   95.58%   +0.02%     
==========================================
  Files         191      190       -1     
  Lines       15794    15806      +12     
==========================================
+ Hits        15093    15108      +15     
+ Misses        701      698       -3

Impacted Files	Coverage Δ
google/cloud/spanner/keys.h	100% <ø> (ø)	⬆️
google/cloud/spanner/samples/samples.cc	90.01% <100%> (ø)	⬆️
google/cloud/spanner/internal/log_wrapper.h	75% <0%> (-9.62%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	80.55% <0%> (-1.86%)	⬇️
google/cloud/spanner/client.cc	96.91% <0%> (-0.62%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	85.55% <0%> (-0.16%)	⬇️
google/cloud/spanner/internal/session_pool.cc	92.24% <0%> (-0.07%)	⬇️
google/cloud/spanner/internal/session.h	100% <0%> (ø)	⬆️
google/cloud/spanner/internal/spanner_stub.cc	77.41% <0%> (ø)	⬆️
...oogle/cloud/spanner/internal/database_admin_stub.h
... and 6 more

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