[LayoutLM] Add clarification to docs (#18716)

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Co-authored-by: Niels Rogge <nielsrogge@Nielss-MacBook-Pro.local>

docs/source/en/model_doc/layoutlm.mdx

@@ -67,7 +67,8 @@ occurs. Those can be obtained using the Python Image Library (PIL) library for e
 ```python
 from PIL import Image
 
-image = Image.open("name_of_your_document - can be a png file, pdf, etc.")
+# Document can be a png, jpg, etc. PDFs must be converted to images.
+image = Image.open(name_of_your_document).convert("RGB")
 
 width, height = image.size
 ```