Improve session window documentation

[suhothayan]

Purpose

Improve usability.

[codecov-io]

Codecov Report

Merging #1580 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1580   +/-   ##
=========================================
  Coverage     67.68%   67.68%           
  Complexity       45       45           
=========================================
  Files           650      650           
  Lines         27471    27471           
  Branches       4330     4330           
=========================================
  Hits          18595    18595           
  Misses         7063     7063           
  Partials       1813     1813

Impacted Files	Coverage Δ	Complexity Δ
...rocessor/stream/window/SessionWindowProcessor.java	66.41% <ø> (ø)	0 <0> (ø)	⬇️
...ut/stream/state/AbsentStreamPreStateProcessor.java	82.11% <0%> (-1.33%)	0% <0%> (ø)
...java/io/siddhi/core/stream/input/InputHandler.java	75% <0%> (+5.55%)	0% <0%> (ø)	⬇️

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