Document functions in src/download.rs with proper rustdocs

[John15321]

This issue focuses on documenting all public functions in src/download.rs using the proper rustdocs format (///).

Action items:

• Review all functions in src/download.rs.
• Add or update documentation comments to follow Rust's standard rustdoc style (///).
• Ensure descriptions are clear and explain parameters, return values, and panics (if any).

Reference: [Tracking] write rustdocs for functions (issue #78)

This issue is scoped only to src/download.rs and does not include other files mentioned in the tracking issue.