doc(pubsub): improve documentation for *AckHandler

[coryan]

From: #9394 (review)

That motivated some other small changes.

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This change is 

[google-cloud-cpp-bot]

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For commit: 883600fb76acd3e9f47268b2616b06820be00af2

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[codecov]

Codecov Report

Merging #9404 (883600f) into main (28610e6) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #9404   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files        1485     1485           
  Lines      136006   136006           
=======================================
+ Hits       128669   128675    +6     
+ Misses       7337     7331    -6     

Impacted Files	Coverage Δ
google/cloud/pubsub/ack_handler.h	80.00% <ø> (ø)
...e/cloud/pubsub/internal/exactly_once_ack_handler.h	70.58% <ø> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
...torage/examples/storage_service_account_samples.cc	97.64% <0.00%> (+0.58%)	⬆️
google/cloud/bigtable/async_read_stream_test.cc	97.99% <0.00%> (+0.66%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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