Allow missing docs on the pin projection constant #1785

Job	Run time
miri	1m 8s
test (1.37)	1m 43s
test (stable)	1m 6s
test (beta)	1m 0s
test (nightly)	2m 6s
check-external-types / check-external-types	26s
deny / deny	16s
docs / docs	14s
msrv / msrv	6m 20s
tidy / tidy	1m 9s
	15m 28s

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