Add a Developer-Comment regarding an unused parameter.

[diesalbla]

Just adds a comment, regarding an unused implicit parameter.

With a link to https://github.com/typelevel/cats/pull/1867/files#r138381991.

[codecov-io]

Codecov Report

Merging #3224 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3224      +/-   ##
==========================================
- Coverage   93.05%   93.03%   -0.02%     
==========================================
  Files         376      376              
  Lines        7412     7412              
  Branches      192      192              
==========================================
- Hits         6897     6896       -1     
- Misses        515      516       +1

Flag	Coverage Δ
#scala_version_212	93.36% <ø> (+0.01%)	⬆️
#scala_version_213	92.81% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Validated.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptyList.scala	98.06% <0%> (-0.65%)	⬇️

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