docs(operators): add documentation for windowWhen

[hardikpthv]

Closes: #127

[codecov-io]

Codecov Report

Merging #291 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #291   +/-   ##
=======================================
  Coverage   90.78%   90.78%           
=======================================
  Files         116      116           
  Lines         445      445           
  Branches        9        9           
=======================================
  Hits          404      404           
  Misses         40       40           
  Partials        1        1

Impacted Files	Coverage Δ
src/operator-docs/transformation/windowWhen.ts	100% <ø> (ø)	:arrow_up:

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[niklas-wortmann]

@hardikpthv I think after the changes requested by ashwin, this pr can be merged. Do you need any help with this one?

[hardikpthv]

Sorry, @JWO719 got busy these days, so couldn't make proposed changes. Let me check this one. I will let you know in case of help. :)

[hardikpthv]

@JWO719, @ashwin-sureshkumar requested changes are made do let me know if anything else is required.