[llvm] [Docs][DebugInfo][RemoveDIs] Document some debug-info transition info (PR #79167)

[J. Ryan Stinnett via llvm-commits]

https://github.com/jryans commented:

Thanks for working on this! It's great to have this overview / summary documentation. 😄 

I've added a few pedantic spelling comments below.

https://github.com/llvm/llvm-project/pull/79167