[clang] [clang] Add reference to documentation of SysVABI (PR #85022)

[via cfe-commits]

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

LGTM. 

Really weird that we somehow both have documentation for it, but also didn’t link to it... if it was meant to be undocumented, we shouldn’t have had any documentation for it just lying around unused either.

https://github.com/llvm/llvm-project/pull/85022