docs(operators): add documentation for toPromise

[niklas-wortmann]

trafficstars

#133

[niklas-wortmann]

Oh.... I will fix the commit message this afternoon. Also I have to change the relatedOperators, which were just copy pasted (hups :D)

[codecov-io]

Codecov Report

Merging #228 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #228   +/-   ##
=======================================
  Coverage   90.49%   90.49%           
=======================================
  Files         115      115           
  Lines         442      442           
  Branches       10       10           
=======================================
  Hits          400      400           
  Misses         40       40           
  Partials        2        2

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

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[niklas-wortmann]

Please don't merge, I would like to add a tip for making use of asnyc await (regarding to #556)

[ashwin-sureshkumar]

@btroncone @sumitarora - Can you please review this ?