Document how a schema can have columns splitted over different files

In the design overview is stated that the format supports that columns of a given schema are splitted over several files . 

To date ,other than the reference in the parquet homepage there are no other references to these feature , not even if it is actually implemented. 

As it currently stands , I believe that for a given row group you cannot have some columns in one file and other column group in another file belonging to the same schema .

If these feature was actually implemented it would open the door to faster data updates which affect a single column in very wide tables, which is typical on big data use cases .

**Reporter**: [Carlos Diogo](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=cdiogo79)

_{**Note**: *This issue was originally created as [PARQUET-1987](https://issues.apache.org/jira/browse/PARQUET-1987). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Commencez par la vue d’ensemble de la conception et la référence existante sur la page d’accueil de Parquet, puis examinez l’implémentation pour déterminer si les colonnes d’un schéma peuvent être réparties entre plusieurs fichiers au sein d’un row group. Documentez le comportement pris en charge ou indiquez clairement la limitation si ce comportement n’est pas implémenté, y compris les éventuelles contraintes pertinentes pour les mises à jour de tables larges.

Rédigé par le modèle d'indexation à partir du texte de l'issue.