Batch scaler docs

[SkafteNicki]

What does this PR do?

Fixes #3780
Updates docs on batch scaler

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[codecov]

Codecov Report

Merging #3894 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3894   +/-   ##
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  Coverage      87%     87%           
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  Files         117     117           
  Lines        9207    9207           
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  Hits         8047    8047           
  Misses       1160    1160