DOC: Removed duplicate returns section

[TomAugspurger]

Closes #20822

[codecov]

Codecov Report

Merging #20823 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #20823      +/-   ##
==========================================
- Coverage   91.78%   91.78%   -0.01%     
==========================================
  Files         153      153              
  Lines       49273    49271       -2     
==========================================
- Hits        45224    45222       -2     
  Misses       4049     4049

Flag	Coverage Δ
#multiple	90.17% <ø> (-0.01%)	⬇️
#single	41.88% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/window.py	96.29% <ø> (-0.01%)	⬇️

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