Enhance Javadoc for AccountMerge

[akankshanimmagadda]

Description

This PR improves the Javadocs in AccountMerge.java to make the class easier to understand and use.

Changes

• Added documentation describing the expected input format.
• Explained how accounts are merged when they share one or more email addresses.
• Clarified the ordering of the merged results returned by mergeAccounts(...).
• Added detailed method-level Javadocs for mergeAccounts(...).
• Added documentation for the internal UnionFind helper class.

Notes

• No functional code was changed.

• This PR only improves documentation and readability.

• 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 algorithms include a corresponding test class that validates their functionality.

• 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 80.25%. Comparing base (9673483) to head (c888f38).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7523      +/-   ##
============================================
- Coverage     80.26%   80.25%   -0.01%     
  Complexity     7357     7357              
============================================
  Files           810      810              
  Lines         23787    23787              
  Branches       4678     4678              
============================================
- Hits          19092    19091       -1     
  Misses         3939     3939              
- Partials        756      757       +1     

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