reworded documentation comment

Author: isaac-mcfadyen

common/common.h

@@ -368,7 +368,7 @@ struct common_params {
     bool use_jinja = false;                                                                                 // NOLINT
     bool enable_chat_template = true;
     common_reasoning_format reasoning_format = COMMON_REASONING_FORMAT_DEEPSEEK;
-    bool prefill_assistant = true;                                                                          // if true, the last server message will be prefilled into the response
+    bool prefill_assistant = true;                                                                          // if true, any trailing assistant message will be prefilled into the response
 
     std::vector<std::string> api_keys;