Bugfixes in KdocComments rule

[petertrr]

What's done:

• Improved logic for merging local and class kdocs of properties
• Improved warning texts in some cases
• Updated diff generator in Test Framework

TODO:

• Before merging, regenerate docs to add the new warning

This pull request closes #964

[codecov]

Codecov Report

Merging #968 (2be64a2) into master (5dd4574) will decrease coverage by 0.02%.
The diff coverage is 79.31%.

@@             Coverage Diff              @@
##             master     #968      +/-   ##
============================================
- Coverage     83.89%   83.86%   -0.03%     
- Complexity     2396     2399       +3     
============================================
  Files           101      101              
  Lines          6054     6063       +9     
  Branches       1789     1790       +1     
============================================
+ Hits           5079     5085       +6     
- Misses          265      268       +3     
  Partials        710      710              

Flag	Coverage Δ
unittests	83.86% <79.31%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...diktat/ruleset/rules/chapter2/kdoc/KdocComments.kt	83.91% <77.77%> (-1.27%)	⬇️
...tlin/org/cqfn/diktat/ruleset/constants/Warnings.kt	98.64% <100.00%> (+<0.01%)	⬆️
...tlin/org/cqfn/diktat/ruleset/utils/KotlinParser.kt	89.28% <100.00%> (ø)

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[kgevorkyan]

lgtm