Fix documentation of linear_only argument in get_incident_variables

[Robbybp]

Fixes

Discrepancy between documentation and behavior

Summary/Motivation:

In #2883, I fixed the behavior of the linear_only argument in get_incident_variables, and claimed that the old behavior (identifying variables that participate linearly even if they also participate nonlinearly) was not my intention. Funnily enough, the documentation of the linear_only option clearly states that variables that are "both linear and nonlinear" will be returned (even though this is mathematically incorrect). This PR fixes the documentation.

Legal Acknowledgement

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4e5121a) 88.09% compared to head (857105a) 88.09%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3067   +/-   ##
=======================================
  Coverage   88.09%   88.09%           
=======================================
  Files         774      774           
  Lines       91537    91537           
=======================================
+ Hits        80636    80644    +8     
+ Misses      10901    10893    -8     

Flag	Coverage Δ
linux	85.45% <ø> (ø)
osx	75.47% <ø> (ø)
other	85.63% <ø> (ø)
win	82.80% <ø> (+<0.01%)	⬆️

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