Add comment explaining TypeToFlatVector<Generic> #11485

Job	Run time
Build	39m 52s
Spark Aggregate Fuzzer	15m 42s
Presto Fuzzer	30m 42s
Presto Bias Fuzzer	0s
Aggregation Fuzzer with Presto as source of truth	17m 55s
Join Fuzzer	17m 30s
Window Fuzzer with Presto as source of truth	17m 39s
RowNumber Fuzzer	15m 37s
Spark Bias Fuzzer	0s
Spark Fuzzer	15m 54s
Signature Changes	4s
Exchange Fuzzer	16m 26s
Biased Aggregation Fuzzer with Presto as source of truth	0s
Writer Fuzzer with Presto as source of truth	19m 54s
	3h 27m 15s

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