Enhance class & function documentation in LFUcache.java

[Hardvan]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.75%. Comparing base (b190cb7) to head (47c6c5b).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5583   +/-   ##
=========================================
  Coverage     54.75%   54.75%           
- Complexity     3453     3454    +1     
=========================================
  Files           538      538           
  Lines         15583    15583           
  Branches       2961     2961           
=========================================
  Hits           8532     8532           
  Misses         6694     6694           
  Partials        357      357           

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