add doctests for example 2/n segmentation

[Borda]

What does this PR do?

Adding doctest for SegModel, follow-up #5079

Before submitting

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[codecov]

Codecov Report

Merging #5083 (499709f) into master (bb6dfb6) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5083   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         134     134           
  Lines        9932    9932           
======================================
- Hits         9246    9239    -7     
- Misses        686     693    +7     

[justusschock]

Sorry @tchaton clicked the wrong button