doc: group client functions

[dbolduc]

#10265 (comment)

unrelated: these should be ///@{ to actually start a group, ditto for closing the group.

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 338970898864372b79ce0fb47b23be9eabeb9385

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Base: 93.81% // Head: 93.81% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (3389708) compared to base (29ffdb9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10268   +/-   ##
=======================================
  Coverage   93.81%   93.81%           
=======================================
  Files        1565     1565           
  Lines      143754   143754           
=======================================
+ Hits       134863   134868    +5     
+ Misses       8891     8886    -5     

Impacted Files	Coverage Δ
...egration_tests/golden/golden_kitchen_sink_client.h	100.00% <ø> (ø)
...tegration_tests/golden/golden_thing_admin_client.h	100.00% <ø> (ø)
generator/internal/client_generator.cc	26.94% <ø> (ø)
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	97.81% <0.00%> (-0.55%)	⬇️
google/cloud/storage/parallel_upload.cc	98.62% <0.00%> (+0.34%)	⬆️
...torage/examples/storage_service_account_samples.cc	97.64% <0.00%> (+0.58%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️
google/cloud/internal/random_test.cc	100.00% <0.00%> (+3.70%)	⬆️
google/cloud/internal/async_connection_ready.cc	93.61% <0.00%> (+4.25%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.