gh-142527: Docs: Clarify that random.seed() discards the sign of an integer input (#142483)

If *a* is an integer, the sign of *a* is discarded in the C source code. Clarify this behavior to prevent foot guns, where a common use case might naively assume that flipping the sign will produce different sequences (e.g. for a train/test split of a synthetic data generator in machine learning).

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>