Add release note

doc/manual/rl-next/forbid-nested-debuggers.md

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+---
+synopsis: Nested debuggers are no longer supported
+prs: 9920
+---
+
+Previously, evaluating an expression that throws an error in the debugger would
+enter a second, nested debugger:
+
+```
+nix-repl> builtins.throw "what"
+error: what
+
+
+Starting REPL to allow you to inspect the current state of the evaluator.
+
+Welcome to Nix 2.18.1. Type :? for help.
+
+nix-repl>
+```
+
+Now, it just prints the error message like `nix repl`:
+
+```
+nix-repl> builtins.throw "what"
+error:
+       … while calling the 'throw' builtin
+         at «string»:1:1:
+            1| builtins.throw "what"
+             | ^
+
+       error: what
+```