[run_clm] clarify why we get the tokenizer warning on long input (#11145)
* clarify why we get the warning here
* Update examples/language-modeling/run_clm.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* wording
* style
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>