Enhance comments and variable initialization in factorial.c

[swahathi]

Description of Change

Added explanatory comments to factorial.c to improve readability and help beginners understand the logic, including handling of large numbers using arrays.

References

N/A

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes: Improved clarity of existing implementation without changing logic.