Add scaladoc example for Semigroupal.product

[ceedubs]

No description provided.

[codecov-io]

Codecov Report

Merging #2427 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2427      +/-   ##
==========================================
+ Coverage   95.22%   95.32%   +0.09%     
==========================================
  Files         351      351              
  Lines        6368     6518     +150     
  Branches      279      301      +22     
==========================================
+ Hits         6064     6213     +149     
- Misses        304      305       +1

Impacted Files	Coverage Δ
core/src/main/scala/cats/Semigroupal.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/foldable.scala	100% <0%> (ø)	⬆️
...rc/main/scala/cats/laws/discipline/Arbitrary.scala	99.22% <0%> (+0.05%)	⬆️
core/src/main/scala/cats/data/Chain.scala	98.09% <0%> (+0.66%)	⬆️

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