[clang] [Docs] Explicitly document libclang ABI and API stability (PR #141657)

[Jannick Kremer via cfe-commits]

https://github.com/DeinAlptraum approved this pull request.

This looks reasonable to me, but I'm coming from the Python bindings anyway and can't comment on the details for the C/C++ side.

https://github.com/llvm/llvm-project/pull/141657