Add `allow(clippy::missing_docs_in_private_items)` #2620

Job	Run time
test (1.56)	1m 35s
test (stable)	53s
test (beta)	53s
test (nightly)	2m 3s
docs / docs	20s
deny / deny	15s
check-external-types / check-external-types	24s
miri / miri	46s
msrv / msrv	4m 2s
tidy / tidy	1m 3s
	12m 14s

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