Some APIs accept both positive and negative indices when accessing items.
This patch changes the documentation for the lower limit from -(count - 1) to
-count to allow accessing the very first item.
For example (content_count = 5):
|first item||last item|
If negative indices are limited to be >= -4 the first item cannot be accessed
using negative indices.
Also, range limit of pack_at is removed for usability.