Nested types: clarify GetCollectionMemory() comment
Nested types: clarify GetCollectionMemory() comment

GetCollectionMemory() returns a TupleRow* defined using the returned
Tuple* so we can easily evaluate conjuncts over ArrayValue tuples
(ArrayValues don't actually contain any TupleRows).
GetCollectionMemory() didn't document how this TupleRow* differs from
that returned by GetMemory() though. See
HdfsParquetScanner::AssembleRows() for how the returned row is
correctly used.

Change-Id: Ibd52a0a742e7868fdaaf9a5a62a630b652b69379
Reviewed-on: http://gerrit.cloudera.org:8080/841
Reviewed-by: Dan Hecht <20ebd99959f0f6e5dcb0fc319b4d8ef53a16b5fb@cloudera.com>
Tested-by: Internal Jenkins

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