refactor: Enhance docs, add more tests in `LFUCache`

[Hardvan]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] 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 :white_check_mark:

Project coverage is 67.43%. Comparing base (b64e53c) to head (0ebb211).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5949      +/-   ##
============================================
+ Coverage     67.40%   67.43%   +0.02%     
- Complexity     4576     4578       +2     
============================================
  Files           620      620              
  Lines         17147    17147              
  Branches       3312     3312              
============================================
+ Hits          11558    11563       +5     
+ Misses         5142     5138       -4     
+ Partials        447      446       -1     

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