Document setindex! updating of coordinates. #1662

Job	Run time
Julia lts - ubuntu-latest - - push	6m 18s
Documentation	5m 45s
Julia pre - macOS-latest - - push	4m 56s
Julia lts - ubuntu-latest - - push	4m 48s
Julia lts - windows-latest - - push	14m 7s
Julia pre - windows-latest - - push	15m 42s
Julia pre - ubuntu-latest - - push	6m 52s
Julia lts - macOS-latest - - push	4m 1s
	1h 2m 29s

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