Fix false positives in UndocumentedPublicProperty

[schalkms]

Properties in classes that are nested, non-public and undocumented are not flagged by this rule anymore.

Related #2580

[codecov-io]

Codecov Report

Merging #2591 into master will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #2591      +/-   ##
============================================
- Coverage     25.20%   25.18%   -0.03%     
  Complexity      394      394              
============================================
  Files           379      379              
  Lines          7418     7425       +7     
  Branches       1225     1227       +2     
============================================
  Hits           1870     1870              
- Misses         5417     5424       +7     
  Partials        131      131              

Impacted Files	Coverage Δ	Complexity Δ
...in/io/gitlab/arturbosch/detekt/rules/Traversing.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../rules/documentation/UndocumentedPublicProperty.kt	17.39% <0.00%> (-0.80%)	1.00 <0.00> (ø)

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