Update docs for diff drive controller

[christophfroehlich]

While having a look on ros-controls/ros2_control_demos#350 I realized, that the diff drive controller documentation was in a rather incomplete state.

I updated it now (and applied another ROS-space-2 flaw)

[codecov-commenter]

Codecov Report

Merging #751 (34356c2) into master (e7f9962) will decrease coverage by 4.90%.
Report is 360 commits behind head on master.
The diff coverage is 25.91%.

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@@            Coverage Diff             @@
##           master     #751      +/-   ##
==========================================
- Coverage   35.78%   30.88%   -4.90%     
==========================================
  Files         189        7     -182     
  Lines       17570      832   -16738     
  Branches    11592      505   -11087     
==========================================
- Hits         6287      257    -6030     
+ Misses        994      133     -861     
+ Partials    10289      442    -9847     

Flag	Coverage Δ
unittests	30.88% <25.91%> (-4.90%)	⬇️

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Files Changed	Coverage Δ
...ontroller/test/test_load_diff_drive_controller.cpp	11.11% <0.00%> (ø)
...ive_controller/test/test_diff_drive_controller.cpp	12.66% <7.81%> (ø)
diff_drive_controller/src/odometry.cpp	42.16% <11.11%> (ø)
diff_drive_controller/src/speed_limiter.cpp	46.55% <11.11%> (ø)
...iff_drive_controller/src/diff_drive_controller.cpp	47.13% <46.94%> (ø)
...de/diff_drive_controller/diff_drive_controller.hpp	100.00% <100.00%> (ø)
...troller/include/diff_drive_controller/odometry.hpp	100.00% <100.00%> (ø)

... and 189 files with indirect coverage changes

[bmagyar]

Thank you for the lots of extra space! :D