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.*}

Inizia dalla panoramica del design e dal riferimento esistente sulla homepage di Parquet, quindi esamina l’implementazione per determinare se le colonne di uno schema possono essere suddivise tra più file all’interno di un row group. Documenta il comportamento supportato oppure indica chiaramente la limitazione se non è implementato, includendo eventuali vincoli rilevanti per gli aggiornamenti di tabelle larghe.

Scritto dal modello di indicizzazione a partire dal testo della issue.