doc: Update the docs about multipart output (#4818)

Signed-off-by: Frost Ming <me@frostming.com>

docs/source/guides/iotypes.rst

@@ -343,16 +343,16 @@ BentoML also supports lists input and output of complex types, such as images an
     @bentoml.service
     class BatchImageService:
         @bentoml.api
-        def enhance_images(self, images: List[PILImage]) -> List[PILImage]:
-            # Process images and return a list of images
+        def enhance_images(self, images: List[PILImage]) -> PILImage:
+            # Process images and return a single image
             ...
 
         @bentoml.api
         def process_files(self, files: List[Path]) -> List[Dict]:
             # Process files and return a list of dictionaries
             ...
 
-Note that currently BentoML does not support output that combines raw binary data (like images or files) with plain dictionary data directly.
+Note that currently BentoML does not support output that contains multiple raw binary data or combines raw binary data (like images or files) with plain dictionary data directly.
 
 Validate data
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