Document web handlers cancellation

[asvetlov]

Fix for #2098

[codecov-io]

Codecov Report

Merging #2257 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2257   +/-   ##
=======================================
  Coverage   97.31%   97.31%           
=======================================
  Files          39       39           
  Lines        7943     7943           
  Branches     1377     1377           
=======================================
  Hits         7730     7730           
  Misses         90       90           
  Partials      123      123

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