Set missing code languages in doc

[BraisGabin]

Improve the documentation a bit

[codecov]

Codecov Report

Merging #2935 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2935      +/-   ##
============================================
+ Coverage     80.23%   80.28%   +0.04%     
- Complexity     2449     2454       +5     
============================================
  Files           421      421              
  Lines          7407     7405       -2     
  Branches       1354     1356       +2     
============================================
+ Hits           5943     5945       +2     
+ Misses          764      757       -7     
- Partials        700      703       +3     

Impacted Files	Coverage Δ	Complexity Δ
...urbosch/detekt/rules/style/UselessCallOnNotNull.kt	76.47% <0.00%> (-4.30%)	13.00% <0.00%> (+5.00%)	⬇️
...gitlab/arturbosch/detekt/generator/out/Markdown.kt	18.18% <0.00%> (-2.51%)	4.00% <0.00%> (ø%)
...ls/src/main/kotlin/io/github/detekt/psi/KtFiles.kt	50.00% <0.00%> (+5.55%)	0.00% <0.00%> (ø%)

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