Add comment where concrete type support is used

[christophebedard]

Similar to #352.

I'm planning to use this function as an example in ros2/ros2_documentation#3762 as well.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.77%. Comparing base (2e491ab) to head (d92f070).
⚠️ Report is 1 commits behind head on rolling.

Additional details and impacted files

@@           Coverage Diff            @@
##           rolling     #353   +/-   ##
========================================
  Coverage    50.77%   50.77%           
========================================
  Files           89       89           
  Lines         2915     2915           
  Branches       846      846           
========================================
  Hits          1480     1480           
  Misses        1338     1338           
  Partials        97       97           

Flag	Coverage Δ
email	50.77% <ø> (?)
rmw-email-cpp	50.77% <ø> (ø)

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