[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
 ```

src/transformers/models/layoutlmv2/feature_extraction_layoutlmv2.py

@@ -168,7 +168,8 @@ class LayoutLMv2FeatureExtractor(FeatureExtractionMixin, ImageFeatureExtractionM
         >>> from transformers import LayoutLMv2FeatureExtractor
         >>> from PIL import Image
 
-        >>> image = Image.open("name_of_your_document - can be a png file, pdf, etc.").convert("RGB")
+        >>> # Document can be a png, jpg, etc. PDFs must be converted to images.
+        >>> image = Image.open(name_of_your_document).convert("RGB")
 
         >>> # option 1: with apply_ocr=True (default)
         >>> feature_extractor = LayoutLMv2FeatureExtractor()

src/transformers/models/layoutlmv3/feature_extraction_layoutlmv3.py

@@ -179,7 +179,8 @@ class LayoutLMv3FeatureExtractor(FeatureExtractionMixin, ImageFeatureExtractionM
         >>> from transformers import LayoutLMv3FeatureExtractor
         >>> from PIL import Image
 
-        >>> image = Image.open("name_of_your_document - can be a png file, pdf, etc.").convert("RGB")
+        >>> # Document can be a png, jpg, etc. PDFs must be converted to images.
+        >>> image = Image.open(name_of_your_document).convert("RGB")
 
         >>> # option 1: with apply_ocr=True (default)
         >>> feature_extractor = LayoutLMv3FeatureExtractor()