docs: added channel amount info to convert_to_grayscale methods in Image and ImageList (#834)

Closes #833 

### Summary of Changes

docs: added channel amount info to `convert_to_grayscale` methods in
`Image` and `ImageList`

Author: Marsmaennchen221

src/safeds/data/image/containers/_image.py

@@ -429,6 +429,9 @@ def convert_to_grayscale(self) -> Image:
         """
         Return a new `Image` that is converted to grayscale.
 
+        The new image will have the same amount of channels as the original image.
+        If you want to change the amount of channels used, please use the method [change_channel][safeds.data.image.containers._image.Image.change_channel].
+
         The original image is not modified.
 
         Returns

src/safeds/data/image/containers/_image_list.py

@@ -876,6 +876,9 @@ def convert_to_grayscale(self) -> ImageList:
         """
         Return a new `ImageList` with all images converted to grayscale.
 
+        The new image list will have the same amount of channels as the original image list.
+        If you want to change the amount of channels used, please use the method [change_channel][safeds.data.image.containers._image_list.ImageList.change_channel].
+
         The original image list is not modified.
 
         Returns