Update the docstring for realtime_publisher.hpp (backport #287)

[mergify]

I looked into RTPublisher code for ros-controls/ros2_controllers#1586. I made some changes to help myself. I'm making this PR just in case this is useful to others.

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This is an automatic backport of pull request #287 done by Mergify.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.76%. Comparing base (6ac84c8) to head (66ff7b4).
Report is 21 commits behind head on humble.

Additional details and impacted files

@@           Coverage Diff           @@
##           humble     #288   +/-   ##
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  Coverage   73.76%   73.76%           
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  Files          10       10           
  Lines         587      587           
  Branches      225      225           
=======================================
  Hits          433      433           
  Misses        104      104           
  Partials       50       50           

Flag	Coverage Δ
unittests	73.76% <ø> (ø)

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Files with missing lines	Coverage Δ
include/realtime_tools/realtime_publisher.hpp	96.82% <ø> (ø)

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