[clang] [llvm] [KeyInstr] Add docs (PR #137991)

[J. Ryan Stinnett via llvm-commits]

https://github.com/jryans commented:

Overall, looks good. Most of my comments are stylistic nits.

I do think it's important to link to this new feature from the "how to updated debug info" page, as without that, pass writers (esp. in the future) are quite unlikely to know they need to do some work for this feature.

https://github.com/llvm/llvm-project/pull/137991