DOC: add limitation of decode_jpeg in the function docstring (#6637)

* add limitation of decode_jpeg in the function docstring

* undo unwanted changes in decode_png
Co-authored-by: Ambuj Pawar <your_email@abc.example>

torchvision/io/image.py

@@ -137,7 +137,9 @@ def decode_jpeg(
             the raw bytes of the JPEG image. This tensor must be on CPU,
             regardless of the ``device`` parameter.
         mode (ImageReadMode): the read mode used for optionally
-            converting the image. Default: ``ImageReadMode.UNCHANGED``.
+            converting the image. The supported modes are: ``ImageReadMode.UNCHANGED``,
+            ``ImageReadMode.GRAY`` and ``ImageReadMode.RGB``
+            Default: ``ImageReadMode.UNCHANGED``.
             See ``ImageReadMode`` class for more information on various
             available modes.
         device (str or torch.device): The device on which the decoded image will