Add note on 'bad features' warning for skewness and kurtosis

[k-dominik]

clarify warning issued for objects that have the same value for all pixels (ref: image.sc topic)

Checklist

• Format code and imports.
• Update user documentation (ilastik.github.io/#269).
• Rebase commits into a logical sequence.
• n/a Add docstrings and comments.
• n/a Add tests.
• n/a Reference relevant issues and other pull requests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.76%. Comparing base (6893110) to head (e28cc36).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2847      +/-   ##
==========================================
- Coverage   55.76%   55.76%   -0.01%     
==========================================
  Files         533      533              
  Lines       62253    62253              
  Branches     8516     8516              
==========================================
- Hits        34717    34714       -3     
- Misses      25781    25786       +5     
+ Partials     1755     1753       -2     

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