Improve some docstrings

[baggepinnen]

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[JuliaControlBot]

This is an automated message.
Plots were compared to references. 3/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

Difference	Reference Image	New Image
✔️ 0.0
✔️ 0.0
✔️ 0.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.52%. Comparing base (8d60c66) to head (0902fe3).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1005   +/-   ##
=======================================
  Coverage   92.52%   92.52%           
=======================================
  Files          41       41           
  Lines        5099     5099           
=======================================
  Hits         4718     4718           
  Misses        381      381           

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