docs(primitives): fix rustdoc for Signature #1861

Job	Run time
fmt	11s
docs	52s
clippy	48s
feature-checks	3m 54s
miri --no-default-features	4m 2s
wasm	40s
miri	3m 37s
miri --all-features	4m 42s
test ubuntu-latest stable --no-default-features	54s
test ubuntu-latest stable	1m 4s
test ubuntu-latest beta --no-default-features	58s
test ubuntu-latest beta	1m 2s
test ubuntu-latest nightly --no-default-features	1m 14s
test ubuntu-latest nightly	1m 19s
test ubuntu-latest 1.65 --no-default-features	35s
test ubuntu-latest 1.65	35s
test windows-latest stable --no-default-features	2m 36s
test windows-latest stable	2m 41s
test windows-latest beta --no-default-features	2m 22s
test windows-latest beta	2m 5s
test windows-latest nightly --no-default-features	2m 48s
test windows-latest nightly	2m 49s
test windows-latest 1.65 --no-default-features	1m 25s
test windows-latest 1.65	1m 26s
test ubuntu-latest 1.65 --features json	44s
test windows-latest 1.65 --features json	1m 30s
test ubuntu-latest stable --all-features	1m 32s
test ubuntu-latest beta --all-features	1m 29s
test ubuntu-latest nightly --all-features	1m 31s
	51m 25s

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