Add function documentation in Sudoku.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 52.97%. Comparing base (be8df21) to head (4c80e7f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5532      +/-   ##
============================================
+ Coverage     52.95%   52.97%   +0.01%     
- Complexity     3309     3310       +1     
============================================
  Files           529      529              
  Lines         15343    15343              
  Branches       2925     2925              
============================================
+ Hits           8125     8128       +3     
+ Misses         6882     6881       -1     
+ Partials        336      334       -2     

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[Hardvan]

I have changed the class and function documentations to reflect the aforementioned suggested changes.