add more doctest to WriterT

[hamednourhani]

Re: Issue #2479

Add more doctest to WriterT methods:

• tell
• written
• value
• listen
• ap
• map
• imap
• mapK
• flatMap
• mapBoth
• bimap
• mapWritten
• swap
• reset
• show
• foldLeft
• foldRight
• traverse

[codecov-io]

Codecov Report

Merging #3073 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3073   +/-   ##
=======================================
  Coverage   93.46%   93.46%           
=======================================
  Files         368      368           
  Lines        6979     6979           
  Branches      184      184           
=======================================
  Hits         6523     6523           
  Misses        456      456

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/WriterT.scala	91.26% <ø> (ø)	⬆️

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[hamednourhani]

Hi @kailuowang would you please review this PR.

[hamednourhani]

@scala-steward would you please review this PR

[kailuowang]

Thanks!

[hamednourhani]

Hi @kailuowang thanks for approving prs. I have some approved pull requests on #2479. Will thay be merged in the next release