Add `allow(clippy::missing_docs_in_private_items)` #2619

Job	Run time
test (1.56)	1m 36s
test (stable)	45s
test (beta)	53s
test (nightly)	1m 54s
deny / deny	16s
docs / docs	18s
check-external-types / check-external-types	18s
miri / miri	51s
msrv / msrv	2m 57s
tidy / tidy	1m 5s
	10m 53s

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