document and clean up algebraic cycles #16644

Job	Run time
doctest (1.10, ubuntu-latest)	26m 33s
test (1.6, short, ubuntu-latest)	1h 27m 18s
doctest (1.11-nightly, ubuntu-latest)	26m 34s
test (1.6, long, ubuntu-latest)	56m 14s
doctest (nightly, ubuntu-latest)	29m 33s
test (1.10, short, ubuntu-latest)	1h 7m 45s
doctest (1.10, ubuntu-latest, depwarn=error)	25m 19s
test (1.10, long, ubuntu-latest)	47m 38s
doctest (1.10, macOS, RPTU)	12m 16s
test (1.11-nightly, short, ubuntu-latest)	1h 22m 56s
test (1.11-nightly, long, ubuntu-latest)	52m 50s
test (nightly, short, ubuntu-latest)	1h 20m 40s
test (nightly, long, ubuntu-latest)	51m 38s
test (1.10, short, ubuntu-latest, depwarn=error)	1h 9m 13s
test (1.10, long, ubuntu-latest, depwarn=error)	15m 44s
test (1.10, short, macOS, RPTU)	2h 30m 15s
test (1.10, long, macOS, RPTU)	14m 8s
test (1.10, book, ubuntu-latest)	2h 30m 16s
	17h 26m 50s

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