warn more against high-order transfer functions

[baggepinnen]

This problem keeps hitting people in one form or another. This PR adds a warning when a high-order tf is created if it uses coefficients of floats that are no more precise than Float64. It also extends the already present section in the docs describing the problem with an illustrative example

[codecov[bot]]

Codecov Report

Merging #800 (b064857) into master (07e0354) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #800   +/-   ##
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  Coverage   97.14%   97.14%           
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  Files           4        4           
  Lines         315      315           
=======================================
  Hits          306      306           
  Misses          9        9           

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[JuliaControlBot]

This is an automated message. Plots were compared to references. 11/11 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
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