Update chat template warnings/guides (#27634)
* Update default ChatML template
* Update docs/warnings
* Update docs/source/en/chat_templating.md
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>
* Slight rework
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Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>
