document and clean up algebraic cycles #16650

Job	Run time
doctest (1.10, ubuntu-latest)	25m 45s
test (1.6, short, ubuntu-latest)	1h 23m 37s
doctest (1.11-nightly, ubuntu-latest)	24m 25s
test (1.6, long, ubuntu-latest)	56m 24s
doctest (nightly, ubuntu-latest)	29m 45s
test (1.10, short, ubuntu-latest)	1h 8m 7s
doctest (1.10, ubuntu-latest, depwarn=error)	25m 3s
test (1.10, long, ubuntu-latest)	45m 57s
doctest (1.10, macOS, RPTU)	12m 19s
test (1.11-nightly, short, ubuntu-latest)	35m 4s
test (1.11-nightly, long, ubuntu-latest)	54m 25s
test (nightly, short, ubuntu-latest)	34m 18s
test (nightly, long, ubuntu-latest)	55m 52s
test (1.10, short, ubuntu-latest, depwarn=error)	36m 28s
test (1.10, long, ubuntu-latest, depwarn=error)	8m 48s
test (1.10, short, macOS, RPTU)	14m 48s
test (1.10, long, macOS, RPTU)	14m 40s
test (1.10, book, ubuntu-latest)	53m 37s
	11h 19m 22s

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