Update debug info assignment tracking docs with current status

[jryans]

I noticed that the assignment tracking docs still say...

Experimental work in progress. Enabling is strongly advised against except for development and testing.

...even though it's been enabled by default since de6da6a (2023-06 / LLVM 17).

@OCHyams, it would be great to update this with the correct current status, and also update the TODO list at the bottom where appropriate.

[llvmbot]

@llvm/issue-subscribers-debuginfo

Author: J. Ryan Stinnett (jryans)

I noticed that the [assignment tracking docs](https://llvm.org/docs/AssignmentTracking.html) still say...

> Experimental work in progress. Enabling is strongly advised against except for development and testing.

...even though it's been enabled by default since de6da6a (2023-06 / LLVM 17).

@OCHyams, it would be great to update this with the correct current status, and also update the TODO list at the bottom where appropriate.

[OCHyams]

Good find, thanks; PR up here #106959.