docs: add FAQ entry for conversation retain format

Addresses common questions from community discussions on the recommended
format and flow for retaining conversations (JSON array vs plain text,
upsert pattern, avoiding pre-summarization).

hindsight-docs/src/pages/faq.md

@@ -238,6 +238,50 @@ Metadata is not a filter — use tags when you need recall to be scoped to a sub
 
 ---
 
+### What is the recommended format for retaining conversations?
+
+Pass the **entire conversation as a single document** and upsert it as the conversation grows — Hindsight chunks it automatically, so you don't need to split it yourself.
+
+**Preferred format: JSON array**
+
+```json
+[
+  {"role": "user",      "content": "I moved to Berlin last month."},
+  {"role": "assistant", "content": "How are you finding it?"},
+  {"role": "user",      "content": "Love it, especially the food scene."}
+]
+```
+
+Hindsight has internal chunking optimizations for the JSON array format, since it's the most common conversation shape.
+
+**Alternative: prefixed plain text**
+
+```
+[2025-06-01T10:32:00Z] user: I moved to Berlin last month.
+[2025-06-01T10:32:05Z] assistant: How are you finding it?
+[2025-06-01T10:32:20Z] user: Love it, especially the food scene.
+```
+
+Adding a username and timestamp prefix to each message improves extraction quality — the LLM uses those signals to attribute facts correctly and reason about timing.
+
+**Use a stable document ID to upsert:**
+
+```python
+await client.retain(
+    bank_id="my-bank",
+    documents=[{
+        "id": "chat-session-abc123",  # stable ID enables upsert
+        "content": conversation,       # full conversation so far
+    }]
+)
+```
+
+Re-retaining with the same `id` replaces the old document and its facts, so you won't accumulate duplicates as the conversation grows.
+
+**Don't pre-summarize or pre-extract facts.** Hindsight does this automatically and needs the full conversation for context — a message like "yes, exactly" or "I'll go with option 2" is meaningless without the surrounding exchange.
+
+---
+
 ## Still have questions?
 
 Join our [Slack community](https://join.slack.com/t/hindsight-space/shared_invite/zt-3nhbm4w29-LeSJ5Ixi6j8PdiYOCPlOgg) or report issues on [GitHub](https://github.com/vectorize-io/hindsight/issues).