Revise explanation of RowMaterializationTimer

IMPALA-8825 added the following explanation of the counter RowMaterializationTimer

/// Tracks the time spent materializing rows and converting them into a QueryResultSet.
/// The QueryResultSet format used depends on the client, for Beeswax clients an ASCII
/// representation is used, whereas for HS2 clients (using TCLIService.thrift) rows are
/// converted into a TRowSet. Materializing rows includes evaluating any yet unevaluated
/// expressions using ScalarExprEvaluators.

This isn't accurate because the actual timer measures time taken running Coordinator::GetNext which can block if any downstream operators in the plan take a long time to materialize rows.

We should revise the explanation of RowMaterializationTimer and consider adding a separate timer that truly measures the amount of time taken to materialize rows (e.g. measure time taken in QueryResultSet::AddRows).