docs: improve LLMCallStartedEvent docstring to clarify multimodal support

Co-Authored-By: Joe Moura <joao@crewai.com>

src/crewai/utilities/events/llm_events.py

@@ -12,7 +12,12 @@ class LLMCallType(Enum):
 
 
 class LLMCallStartedEvent(CrewEvent):
-    """Event emitted when a LLM call starts"""
+    """Event emitted when a LLM call starts
+    
+    Attributes:
+        messages: Content can be either a string or a list of dictionaries that support
+            multimodal content (text, images, etc.)
+    """
 
     type: str = "llm_call_started"
     messages: Union[str, List[Dict[str, Any]]]