Better warning message

Signed-off-by: Alec Solder <alecs@fb.com>

Author: Alec Solder

vllm/config/structured_outputs.py

@@ -78,9 +78,10 @@ def _validate_structured_output_config(self) -> Self:
             and self.reasoning_parser not in valid_reasoning_parsers
         ):
             logger.warning(
-                "Reasoning parser %s not defined in reasoning_parser_plugin "
-                "argument. Assuming it is registered to the "
-                "ReasoningParserManager programmatically.",
+                "Reasoning parser %s not found among built-in parsers or "
+                "reasoning_parser_plugin argument. Assuming it will be "
+                "registered to the ReasoningParserManager programmatically "
+                "before use.",
                 self.reasoning_parser,
             )