[llvm] [NFC] Fix #106873 - update assignment tracking docs (PR #106959)

[Jeremy Morse via llvm-commits]

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

LGTM as a status update; IIRC we didn't turn it on everywhere due to the performance implications in LTO? Now that we've radically changed the storage for variable debug-info, perhaps that's now something we can revisit.

https://github.com/llvm/llvm-project/pull/106959