Fix foldA, reduceA, and reduceMapA short-circuiting

[travisbrown]

As discussed in #3150, the only real reason I can see for foldA and reduceA to exist is because they can short-circuit, while fold(fga)(Applicative.monoid) etc. can't. This change fixes them so they actually do short-circuit, and adds some tests to verify this.

[travisbrown]

I went ahead and added the API docs edits in this PR, hope you don't mind, @LukaJCB.

[codecov-io]

Codecov Report

Merging #3199 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3199   +/-   ##
=======================================
  Coverage   93.06%   93.06%           
=======================================
  Files         376      376           
  Lines        7409     7409           
  Branches      197      191    -6     
=======================================
  Hits         6895     6895           
  Misses        514      514

Flag	Coverage Δ
#scala_version_212	93.39% <100%> (ø)	⬆️
#scala_version_213	92.83% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/Foldable.scala	98.5% <100%> (ø)	⬆️
core/src/main/scala/cats/Reducible.scala	100% <100%> (ø)	⬆️

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