[docs] Mention how to get/set a bigint PyLong via the C API (GH-101270)

We don't need direct C APIs to get at a bigint representation of PyLong but we
do want the few people who need to understand how.

Additional Author:  CAM-Gerlach
(cherry picked from commit e244401ce508ad391295beb636e499fcc6797a2a)

Co-authored-by: Gregory P. Smith <greg@krypto.org>