add docs for polygonToCellsExperimental #1631

Job	Run time
Test Compile clang Debug	3m 32s
CMake correctness checks	6s
Coverage	3m 55s
Test Compile clang Release	3m 21s
Test Compile clang Debug -fsanitize=undefined,float-divide-by-zero -fno-sanitize-recover=undefined,float-divide-by-zero	6m 25s
Test Compile gcc Debug	3m 39s
Test Compile clang Release -fsanitize=undefined,float-divide-by-zero -fno-sanitize-recover=undefined,float-divide-by-zero	3m 14s
Test Compile gcc Release	2m 35s
Test Valgrind	13m 41s
Test Compile clang Debug -fsanitize=memory -fno-sanitize-recover=memory	34m 14s
Test Compile clang Release -fsanitize=memory -fno-sanitize-recover=memory	5m 25s
Test Compile clang Debug -fsanitize=address -fno-sanitize-recover=address	5m 52s
Test Compile clang Release -fsanitize=address -fno-sanitize-recover=address	3m 25s
	1h 29m 24s

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