Outer Documentation Comments

[rkdud007]

For better readability and usage for potential developers who might utilize accumulator library + also for inner documentation purpose, need to update all the outer/inner documentation comments by refer to top tier rust opensource library projects.

For simple example, every public functions should have like the comment below:

/// Adds two numbers together.
///
/// # Arguments
///
/// * `x` - The first number to add.
/// * `y` - The second number to add.
///
/// # Examples
///
/// ```
/// let result = add(2, 3);
/// assert_eq!(result, 5);
/// ```
///
/// # Returns
///
/// This function returns the sum of `x` and `y`.
///