[llvm] [DebugInfo][RemoveDIs] Add documentation for updating code to handle debug records (PR #93562)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> What's the status of #91725? That seems to update some of the same text (at least the summary paragraph at the top). Maybe it was forgotten? Perhaps best to merge that first and then rebase this to avoid conflicts.

That patch has been deliberately left hanging until the [corresponding textual IR update](https://github.com/llvm/llvm-project/pull/91724) has landed/is about to land, just in case any change to the approach becomes necessary before then. It's more likely that this patch will land first, because _this_ patch exists to assist with https://github.com/llvm/llvm-project/pull/89799, which is currently reverted and waiting for a reduced reproducer, and the IR patch is blocked on that patch. So in short, it's just a dependency tree situation; this change should really have been made before landing the aforementioned PR in the first place.

https://github.com/llvm/llvm-project/pull/93562