docs(notebooks): add DETACH DELETE rationale in langchain cleanup cell

Author: polaz

demo/notebooks/02_langchain_graph_chain.ipynb

@@ -279,7 +279,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "graph.query(\"MATCH (n) WHERE n.name ENDS WITH $tag DETACH DELETE n  # atomically removes edges before deleting node\", params={\"tag\": tag})\nprint(\"Cleaned up\")\ngraph.close()\nclient.close()  # injected client — owned by caller"
+    "# DETACH DELETE atomically removes all edges then the node in one operation.\n# Two-step MATCH (n)-[r]-() / DELETE r / DELETE n is avoided because an\n# undirected MATCH returns each edge from both endpoints, so the second pass\n# fails with \"cannot delete node with connected edges\".\ngraph.query(\"MATCH (n) WHERE n.name ENDS WITH $tag DETACH DELETE n\", params={\"tag\": tag})\nprint(\"Cleaned up\")\ngraph.close()\nclient.close()  # injected client — owned by caller"
    ]
   }
  ],