Merge pull request #2799 from martinRenou/add_file_upload_warning_docs

Docs: Add warning concerning the FileUpload widget

Author: jasongrout

docs/source/examples/Widget List.ipynb

@@ -1130,6 +1130,10 @@
     "- The `.value` traitlet is now a list of dictionaries, rather than a dictionary mapping the uploaded name to the content. To retrieve the original form, use `{f[\"name\"]: f.content.tobytes() for f in uploader.value}`.\n",
     "- The `.data` traitlet has been removed. To retrieve it, use `[f.content.tobytes() for f in uploader.value]`.\n",
     "- The `.metadata` traitlet has been removed. To retrieve it, use `[{k: v for k, v in f.items() if k != \"content\"} for f in w.value]`.\n",
+    "</div>\n",
+    "\n",
+    "<div class=\"alert alert-warning\">\n",
+    "Warning: When using the `FileUpload` Widget, uploaded file content might be saved in the notebook if widget state is saved.\n",
     "</div>"
    ]
   },