add more doctest to Ior

[hamednourhani]

Re: Issue #2479

Add more doctest to Ior methods:

• fold
• putLeft
• putRight
• isLeft
• isRight
• isBoth
• left
• right
• onlyLeft
• onlyRight
• onlyRightOrLeft
• onlyBoth
• pad
• unwrap
• toIorNes
• toIorNel
• toEither
• toValidated
• toOption
• toList
• to
• swap
• exists
• forall
• getOrElse
• valueOr
• bimap
• leftMap
• map
• flatMap
• foldLeft
• combine

[codecov-io]

Codecov Report

Merging #3063 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3063      +/-   ##
==========================================
+ Coverage   93.46%   93.48%   +0.01%     
==========================================
  Files         368      368              
  Lines        6979     6979              
  Branches      184      184              
==========================================
+ Hits         6523     6524       +1     
+ Misses        456      455       -1

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Ior.scala	98.52% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/semigroup.scala	75% <0%> (+25%)	⬆️

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[hamednourhani]

Hi @kailuowang . would you please review this PR.
And one question : what is the process of adding PRs to merged branches

[kailuowang]

Thanks very much!