Add missing Abelian groups docs #16871

Job	Run time
doctest (1.10, ubuntu-latest)	26m 3s
doctest (1.11-nightly, ubuntu-latest)	24m 30s
test (1.6, short, ubuntu-latest)	1h 48m 11s
doctest (nightly, ubuntu-latest)	24m 11s
test (1.6, long, ubuntu-latest)	55m 27s
doctest (1.10, ubuntu-latest, depwarn=error)	26m 5s
test (1.10, short, ubuntu-latest)	1h 11m 48s
doctest (1.10, macOS, RPTU)	12m 16s
test (1.10, long, ubuntu-latest)	44m 27s
test (1.11-nightly, short, ubuntu-latest)	1h 26m 5s
test (1.11-nightly, long, ubuntu-latest)	51m 46s
test (nightly, short, ubuntu-latest)	1h 21m 23s
test (nightly, long, ubuntu-latest)	54m 41s
test (1.10, short, ubuntu-latest, depwarn=error)	1h 10m 50s
test (1.10, long, ubuntu-latest, depwarn=error)	45m 47s
test (1.10, short, macOS, RPTU)	7m 23s
test (1.10, long, macOS, RPTU)	13m 52s
test (1.10, book, ubuntu-latest)	48m 51s
test (nightly, book, ubuntu-latest)	1h 5m 14s
	15h 18m 50s

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