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

[J. Ryan Stinnett via llvm-commits]

================
@@ -0,0 +1,106 @@
+# What's all this then?
+
+We're planning on removing debug info intrinsics from LLVM, as they're slow, unwieldy and can confuse optimisation passes if they're not expecting them. Instead of having a sequence of instructions that looks like this:
+
+```text
+    %add = add i32 %foo, %bar
+    call void @llvm.dbg.value(metadata %add, ...
+    %sub = sub i32 %add, %tosub
+    call void @llvm.dbg.value(metadata %sub, ...
+    call void @a_normal_function()
+```
+
+with dbg.value intrinsics representing debug info records, it would instead be printed as:
----------------
jryans wrote:

```suggestion
with `dbg.value` intrinsics representing debug info records, it would instead be printed as:
```

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