Refresh inline documentation

[lexidor]

Some of the docblocks have drifted away from the implementation.
The most common cause of this drift is functions that have been changed to take Containers instead of Traversables.

/**
 * Returns a new vec containing the subsequence of the given Traversable
 * determined by the offset and length.
 *
 * If no length is given or it exceeds the upper bound of the Traversable,
 * the vec will contain every element after the offset.
 *
 * - To take only the first `$n` elements, see `Vec\take()`.
 * - To drop the first `$n` elements, see `Vec\drop()`.
 *
 * Time complexity: O(n), where n is the size of the slice
 * Space complexity: O(n), where n is the size of the slice
 */
<<__Rx>>
function slice<Tv>(
  Container<Tv> $container, // I am Container<T>, not Traversable<T>
  int $offset,
  ?int $length = null,
): vec<Tv>;

I will have a look at this later, when I have some time to kill.