Fix steering_controllers_library docs and msg field

[christophfroehlich]

Fixes #1732

Needs ros-controls/control_msgs#211

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.74%. Comparing base (4612b48) to head (ad333de).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1733      +/-   ##
==========================================
- Coverage   85.74%   85.74%   -0.01%     
==========================================
  Files         123      123              
  Lines       11916    11915       -1     
  Branches     1017     1017              
==========================================
- Hits        10217    10216       -1     
  Misses       1377     1377              
  Partials      322      322              

Flag	Coverage Δ
unittests	85.74% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...roller/test/test_ackermann_steering_controller.cpp	100.00% <100.00%> (ø)
...ntroller/test/test_bicycle_steering_controller.cpp	100.00% <ø> (ø)
...llers_library/src/steering_controllers_library.cpp	70.37% <100.00%> (ø)
...troller/test/test_tricycle_steering_controller.cpp	100.00% <100.00%> (ø)

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[bmagyar]

please merge when the semi-binary CI is happy

[saikishor]

LGTM
Thanks