some improvements to design docs

[baggepinnen]

No description provided.

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 8/11 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
❌ 0.048
✔️ 0.0
✔️ 0.001
✔️ 0.001
✔️ 0.0
❌ 0.043
❌ 0.059
✔️ 0.01

[codecov]

Codecov Report

Merging #698 (4771e20) into master (d2391ee) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #698   +/-   ##
=======================================
  Coverage   87.58%   87.58%           
=======================================
  Files          35       35           
  Lines        3601     3601           
=======================================
  Hits         3154     3154           
  Misses        447      447           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d2391ee...4771e20. Read the comment docs.