Add comment explaining TypeToFlatVector<Generic> (#11187) #11507

Job	Run time
Build	39m 38s
Presto Fuzzer	30m 44s
Spark Aggregate Fuzzer	15m 41s
Exchange Fuzzer	16m 40s
Aggregation Fuzzer with Presto as source of truth	17m 23s
Presto Bias Fuzzer	0s
Biased Aggregation Fuzzer with Presto as source of truth	0s
Join Fuzzer	17m 28s
Window Fuzzer with Presto as source of truth	17m 28s
Writer Fuzzer with Presto as source of truth	17m 18s
RowNumber Fuzzer	15m 38s
Spark Fuzzer	15m 46s
Spark Bias Fuzzer	0s
Signature Changes	2s
	3h 23m 46s

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