Document PyLong_AS_LONG

[encukou]

PyLong_AS_LONG is undocumented. It should be documented, so people know how to replace it.
See also: capi-workgroup/decisions#38

Linked PRs

• gh-124385: Document and soft-deprecate PyLong_AS_LONG #124386
• [3.13] gh-124385: Document and soft-deprecate PyLong_AS_LONG (GH-124386) #124719
• [3.12] gh-124385: Document and soft-deprecate PyLong_AS_LONG (GH-124386) #130549