docs: Add documentation for new multiline tag input method

aider/website/_includes/multi-line.md

@@ -1,5 +1,14 @@
 You can send long, multi-line messages in the chat in a few ways:
   - Paste a multi-line message directly into the chat.
   - Enter `{` alone on the first line to start a multiline message and `}` alone on the last line to end it.
+  - Enter `{tag` (where "tag" is any sequence of letters/numbers) and end with `tag}`. This is useful when you need to include closing braces `}` in your message.
   - Use Meta-ENTER to start a new line without sending the message (Esc+ENTER in some environments).
   - Use `/paste` to paste text from the clipboard into the chat.
+
+Example with a tag:
+```
+{python
+def hello():
+    print("Hello}")  # Note: contains a brace
+python}
+```