add docs for polygonToCellsExperimental #1607

Job	Run time
Test Compile clang Debug	3m 31s
Test Compile clang Release	2m 30s
Test Compile clang Debug -fsanitize=undefined,float-divide-by-zero -fno-sanitize-recover=undefined,float-divide-by-zero	6m 19s
CMake correctness checks	6s
Coverage	3m 34s
Test Compile clang Release -fsanitize=undefined,float-divide-by-zero -fno-sanitize-recover=undefined,float-divide-by-zero	3m 5s
Test Compile clang Debug -fsanitize=memory -fno-sanitize-recover=memory	31m 9s
Test Compile clang Release -fsanitize=memory -fno-sanitize-recover=memory	6m 0s
Test Compile gcc Debug	3m 31s
Test Compile clang Debug -fsanitize=address -fno-sanitize-recover=address	5m 35s
Test Compile gcc Release	2m 21s
Test Compile clang Release -fsanitize=address -fno-sanitize-recover=address	3m 18s
Test Valgrind	14m 8s
	1h 25m 7s

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