Adjust stable ABI internal documentation (GH-96896)

I was perusing this file, and noticed that this part of the documentation is slightly out of date: the `struct` items in this TOML file currently contain `struct_abi_kind` members, which distinguish between the different types of ABI compatibility described in the comment.

I've updated the comment to reflect this.
(cherry picked from commit 6e533088290b909df324615df24286489603989f)

Co-authored-by: William Woodruff <william@yossarian.net>