Mention choices behavior

examples/How_to_stream_completions.ipynb

@@ -574,7 +574,10 @@
    "source": [
     "### 4. How to get token usage data for streamed chat completion response\n",
     "\n",
-    "You can get token usage data for your streamed response by setting `stream_options={\"include_usage\": True}`. When you do so, an extra chunk will be returned as the final chunk. You can access the usage data for the whole response via the `usage` field on this chunk. Note that all other chunks will also have a `usage` field, but they all contain a null value.\n",
+    "You can get token usage statistics for your streamed response by setting `stream_options={\"include_usage\": True}`. When you do so, an extra chunk will be streamed as the final chunk. You can access the usage data for the entire request via the `usage` field on this chunk. A few important notes when you set `stream_options={\"include_usage\": True}`:\n",
+    "* The value for the `usage` field on all chunks except for the last one will be null.\n",
+    "* The `usage` field on the last chunk contains token usage statistics for the entire request.\n",
+    "* The `choices` field on the last chunk will always be an empty array `[]`.\n",
     "\n",
     "Let's see how it works using the example in 2."
    ]