DOC: precise CQP constraints on property names

Current description is on the FAQ :
https://groupes.renater.fr/wiki/txm-users/public/faq#quel_est_le_format_du_fichier_metadatacsv

Can we be more precise about punctuation characters ?
It would be useful to have 1 ou 2 such characters (e.g. hyphen) for the clarity of property names.

(from redmine: issue id 1019, created on 2014/09/26 by Benedicte Pincemin)