docs: add docstrings to list_user_courses.rs

[Majormaxx]

Summary

This PR adds comprehensive docstrings to all functions in contracts/course/course_access/src/functions/list_user_courses.rs, following the project's documentation template as specified in issue #203.

Changes

list_user_courses function

Added documentation including:

• Brief description: Explains that the function retrieves the list of courses associated with a specific user
• Arguments:
  • env - The Soroban environment context
  • user - The blockchain address of the user
• Returns: Description of the UserCourses struct returned

test_list_user_courses test function

Added documentation describing the test's purpose and what it verifies.

Documentation Format

All docstrings follow the required template:

/// Brief description: explains what the function does.
///
/// # Arguments
///
/// * `param1` - Explanation of the first parameter.
/// * `param2` - Explanation of the second parameter.
///
/// # Returns
///
/// * `Result<T, E>` - What the function returns.

Checklist

• Docstrings added to all functions
• Follows project documentation template
• Includes parameter descriptions
• Includes return value description

Closes #203