fix: don't overwrite doclet description if already present (@desc tag)

[Kocal]

Fix #140

[codecov-io]

Codecov Report

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The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #141   +/-   ##
=========================================
  Coverage          ?   70.83%           
=========================================
  Files             ?        5           
  Lines             ?       48           
  Branches          ?       11           
=========================================
  Hits              ?       34           
  Misses            ?       11           
  Partials          ?        3

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