[doc] Clarify selector prompt for SelectorGroupChat (microsoft#6399)

Resolves microsoft#6383

Author: ekzhu

python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_selector_group_chat.py

@@ -328,6 +328,9 @@ class SelectorGroupChat(BaseGroupChat, Component[SelectorGroupChatConfig]):
         max_turns (int, optional): The maximum number of turns in the group chat before stopping. Defaults to None, meaning no limit.
         selector_prompt (str, optional): The prompt template to use for selecting the next speaker.
             Available fields: '{roles}', '{participants}', and '{history}'.
+            `{participants}` is the names of candidates for selection. The format is `["<name1>", "<name2>", ...]`.
+            `{roles}` is a newline-separated list of names and descriptions of the candidate agents. The format for each line is: `"<name> : <description>"`.
+            `{history}` is the conversation history formatted as a double newline separated of names and message content. The format for each message is: `"<name> : <message content>"`.
         allow_repeated_speaker (bool, optional): Whether to include the previous speaker in the list of candidates to be selected for the next turn.
             Defaults to False. The model may still select the previous speaker -- a warning will be logged if this happens.
         max_selector_attempts (int, optional): The maximum number of attempts to select a speaker using the model. Defaults to 3.

python/packages/autogen-core/docs/src/user-guide/agentchat-user-guide/selector-group-chat.ipynb

@@ -272,6 +272,11 @@
             "cell_type": "markdown",
             "metadata": {},
             "source": [
+                "The string variables available in the selector prompt are:\n",
+                "- `{participants}`: The names of candidates for selection. The format is `[\"<name1>\", \"<name2>\", ...]`.\n",
+                "- `{roles}`: A newline-separated list of names and descriptions of the candidate agents. The format for each line is: `\"<name> : <description>\"`.\n",
+                "- `{history}`: The conversation history formatted as a double newline separated of names and message content. The format for each message is: `\"<name> : <message content>\"`.\n",
+                "\n",
                 "```{tip}\n",
                 "Try not to overload the model with too much instruction in the selector prompt.\n",
                 "\n",
@@ -1018,7 +1023,7 @@
             "name": "python",
             "nbconvert_exporter": "python",
             "pygments_lexer": "ipython3",
-            "version": "3.12.9"
+            "version": "3.12.3"
         }
     },
     "nbformat": 4,