Test

Add documentation for the behaviors for aggregates #950

Job	Run time
PostGraphile mutation tests (exported as graphql-js) / build (12, 20.x)	8m 45s
@dataplan/pg tests / build (12, 20.x)	3m 34s
PostGraphile mutation tests / build (12, 20.x)	3m 14s
PostGraphile mutation tests (exported as typeDefs) / build (12, 20.x)	7m 52s
PostGraphile polymorphic query tests / build (12, 20.x)	3m 17s
PostGraphile polymorphic query tests (exported as graphql-js) / build (12, 20.x)	7m 28s
PostGraphile polymorphic query tests (exported as typeDefs) / build (12, 20.x)	6m 6s
PostGraphile v4 queries tests / build (12, 20.x)	3m 48s
PostGraphile v4 queries tests (exported as graphql-js) / build (12, 20.x)	16m 6s
PostGraphile v4 queries tests (exported as typeDefs) / build (12, 20.x)	12m 1s
PostGraphile schema tests / build (12, 20.x)	6m 5s
Small package tests / build (12, 20.x)	3m 9s
	1h 21m 25s

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