add doctest for #20846

[mezzarobba]

Fixes #20846 (but not the related #17400).

[fchapoton]

ok

[codecov-commenter]

Codecov Report

Base: 88.59% // Head: 88.59% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (0e9cd40) compared to base (66b319b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #35001      +/-   ##
===========================================
- Coverage    88.59%   88.59%   -0.01%     
===========================================
  Files         2136     2136              
  Lines       396141   396141              
===========================================
- Hits        350948   350943       -5     
- Misses       45193    45198       +5     

Impacted Files	Coverage Δ
src/sage/interfaces/qsieve.py	71.30% <0.00%> (-2.61%)	⬇️
src/sage/modular/local_comp/liftings.py	98.33% <0.00%> (-1.67%)	⬇️
src/sage/cpython/_py2_random.py	74.79% <0.00%> (-1.24%)	⬇️
src/sage/schemes/elliptic_curves/hom_frobenius.py	96.34% <0.00%> (-1.22%)	⬇️
src/sage/schemes/elliptic_curves/hom_velusqrt.py	94.09% <0.00%> (-1.19%)	⬇️
src/sage/coding/channel.py	97.10% <0.00%> (-0.73%)	⬇️
...e/combinat/cluster_algebra_quiver/mutation_type.py	50.35% <0.00%> (-0.60%)	⬇️
src/sage/graphs/generators/random.py	92.12% <0.00%> (-0.52%)	⬇️
src/sage/quadratic_forms/binary_qf.py	92.78% <0.00%> (-0.50%)	⬇️
...ge/schemes/elliptic_curves/weierstrass_morphism.py	93.01% <0.00%> (-0.44%)	⬇️
... and 20 more

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