Clarify use of RowGroup.ordinal field

[ggershinsky]

Describe the enhancement requested

Encrypted files use three types of ordinals: row group, column, page. All three are simple local counters in both writers and readers. In addition, the row group ordinal is stored in the parquet footer (RowGroup.ordinal field). Parquet implementors can benefit from a clarification on the reason for and intended use of this field.