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.*}