Clarify withStateHandlers behavior

[idris]

Closes #515

[codecov-io]

Codecov Report

Merging #518 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #518   +/-   ##
=======================================
  Coverage   88.94%   88.94%           
=======================================
  Files          53       53           
  Lines         389      389           
=======================================
  Hits          346      346           
  Misses         43       43

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[istarkov]

Thank you!