Add documentation for FlatMap and Bifunctor

[asoltysik]

No description provided.

[LukaJCB]

This should be Bifunctor I assume? :)

[johnynek]

Tuple2 is very similar to Map, maybe mention that one first in FlatMap and mention Map as a similar case.

[codecov-io]

Codecov Report

Merging #2459 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2459   +/-   ##
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  Coverage   95.39%   95.39%           
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  Files         357      357           
  Lines        6516     6516           
  Branches      288      288           
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  Hits         6216     6216           
  Misses        300      300

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[kailuowang]

Tuple2 is a good example to add that as well. I propose we merge and add that in a separate PR.