feat(generator): document retry policies

[coryan]

Part of the work for #9434

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This change is 

[coryan]

@dbolduc there are some Bigtable changes here that you may want to review.

[codecov]

Codecov Report

Patch coverage: 71.03% and project coverage change: -0.03 ⚠️

Comparison is base (286a6eb) 93.71% compared to head (265c2c2) 93.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12024      +/-   ##
==========================================
- Coverage   93.71%   93.68%   -0.03%     
==========================================
  Files        2010     2013       +3     
  Lines      174761   174904     +143     
==========================================
+ Hits       163775   163862      +87     
- Misses      10986    11042      +56     

Impacted Files	Coverage Δ
generator/internal/connection_generator.cc	37.03% <ø> (-0.47%)	⬇️
google/cloud/bigtable/rpc_retry_policy_test.cc	97.97% <ø> (-0.12%)	⬇️
google/cloud/bigtable/testing/mock_policies.h	66.66% <0.00%> (ø)
google/cloud/internal/retry_policy_impl.h	100.00% <ø> (ø)
google/cloud/pubsub/schema_connection.h	62.50% <ø> (-37.50%)	⬇️
generator/internal/descriptor_utils.cc	84.50% <18.18%> (-3.08%)	⬇️
...tion_tests/golden/v1/golden_rest_only_connection.h	58.33% <56.52%> (-41.67%)	⬇️
...n_tests/golden/v1/golden_kitchen_sink_connection.h	78.57% <77.77%> (-21.43%)	⬇️
generator/integration_tests/tests/plumbing_test.cc	92.30% <83.33%> (-1.25%)	⬇️
...on_tests/golden/v1/golden_thing_admin_connection.h	85.71% <85.18%> (-14.29%)	⬇️
... and 5 more

... and 5 files with indirect coverage changes

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[dbolduc]

@dbolduc there are some Bigtable changes here that you may want to review.

changes in google/cloud/bigtable/... LGTM