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

[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 70.15%. Comparing base (2ec4a1f) to head (8c1f6d3).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5858      +/-   ##
============================================
- Coverage     70.15%   70.15%   -0.01%     
  Complexity     4760     4760              
============================================
  Files           635      635              
  Lines         17322    17322              
  Branches       3333     3333              
============================================
- Hits          12153    12152       -1     
- Misses         4708     4709       +1     
  Partials        461      461              

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