Add documentation for new type hints in Koalas

[HyukjinKwon]

This PR adds some documentations for #1578

[codecov-commenter]

Codecov Report

Merging #1584 into master will increase coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1584      +/-   ##
==========================================
+ Coverage   94.25%   94.57%   +0.31%     
==========================================
  Files          38       38              
  Lines        8691     8789      +98     
==========================================
+ Hits         8192     8312     +120     
+ Misses        499      477      -22     

Impacted Files	Coverage Δ
databricks/koalas/usage_logging/__init__.py	91.37% <0.00%> (-3.27%)	⬇️
databricks/koalas/missing/__init__.py	100.00% <0.00%> (ø)
databricks/koalas/config.py	99.00% <0.00%> (+0.01%)	⬆️
databricks/koalas/utils.py	96.38% <0.00%> (+0.01%)	⬆️
databricks/koalas/series.py	97.63% <0.00%> (+0.01%)	⬆️
databricks/koalas/indexes.py	96.86% <0.00%> (+0.03%)	⬆️
databricks/koalas/generic.py	96.71% <0.00%> (+0.06%)	⬆️
databricks/koalas/plot.py	94.36% <0.00%> (+0.08%)	⬆️
databricks/koalas/indexing.py	92.25% <0.00%> (+0.08%)	⬆️
databricks/koalas/base.py	97.37% <0.00%> (+0.09%)	⬆️
... and 9 more

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[itholic]

Looks really good to me !
(And also, how about we merge this and #1589 right after 1.0.0 will be actually released ??)

[HyukjinKwon]

The documentation should be the part of the release so it should include.

[itholic]

@HyukjinKwon Ah, makes sense.

[HyukjinKwon]

I am merging this to unblock the release.