[Documentation] Warn that DataCollatorForWholeWordMask is limited to BertTokenizer-like tokenizers
Author: ionicsolutions
Pull request: yes

# What does this PR do?

Currently, the `DataCollatorForWholeWordMasking` added with #7925 only works for one specific family of tokenizers, but the documentation does not mention this nor is the user warned when using this data collator with an incompatible tokenizer. Since the data collator will run with all tokenizers, just not produce the desired output, this is very misleading for users. 

This PR adds a note to the documentation and a warning that is issued when a user attempts to create the whole word mask with a (presumably) incompatible tokenizer.

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Fixes #11768


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