Document behavior of unions containing unions #527

Job	Run time
JS Tests (16.x)	2m 34s
Lint (18.x)	34s
JS Tests (18.x)	2m 28s
Lint (19.x)	30s
Build and run integration tests (18.x)	2m 44s
JS Tests (19.x)	2m 34s
Lint (20.x)	29s
Build and run integration tests (19.x)	2m 43s
JS Tests (20.x)	2m 26s
Lint (21.x)	28s
Build and run integration tests (20.x)	2m 28s
JS Tests (21.x)	2m 25s
Build and run integration tests (21.x)	2m 22s
Publish to NPM	32s
	25m 17s

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