Thank you @Jarvis.
I added this code at the end of the functions.php file of the main theme of the website, and replaced every “extra_description” characters strings by the name of my field (“contenu_categorie”) (except in the 2 first lines of the code of course).
… Nothing has changed in the front end, even if I clear the cache (of course, I added some contents in my fields from the back-office)…
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