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

[J. Ryan Stinnett via llvm-commits]

https://github.com/jryans commented:

The TODO list still says it's an "experimental work in progress", which reads a bit strangely for something that's now enabled by default. Perhaps reword that bit as well?

I assume the rest of the TODO list is still valid, since you haven't removed any items here...?

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