docs: clarify discrepancy in ToolCallPart.part_kind and ToolCallPartDelta.part_delta_kind (#4499)

Author: advaitathreya

pydantic_ai_slim/pydantic_ai/messages.py

@@ -1448,7 +1448,7 @@ class ToolCallPart(BaseToolCallPart):
     _: KW_ONLY
 
     part_kind: Literal['tool-call'] = 'tool-call'
-    """Part type identifier, this is available on all parts as a discriminator."""
+    """Part type identifier, this is available on all parts as a discriminator. Note that this is different from `ToolCallPartDelta.part_delta_kind`."""
 
 
 @dataclass(repr=False)
@@ -1926,7 +1926,7 @@ class ToolCallPartDelta:
     """
 
     part_delta_kind: Literal['tool_call'] = 'tool_call'
-    """Part delta type identifier, used as a discriminator."""
+    """Part delta type identifier, used as a discriminator. Note that this is different from `ToolCallPart.part_kind`."""
 
     def as_part(self) -> ToolCallPart | None:
         """Convert this delta to a fully formed `ToolCallPart` if possible, otherwise return `None`.