docs: Remove #[doc(hidden)] and add documentation for internal traits #620

Job	Run time
format/nightly	17s
miri	10m 42s
Pass MSRV values to other jobs	2s
nostd/thumbv6m-none-eabi/stable	19s
cargo-careful	1m 15s
cross_test/i686-unknown-linux-gnu/stable	1m 22s
cross_test/s390x-unknown-linux-gnu/stable	1m 20s
clippy/stable	22s
docs/nightly	22s
blas-msrv	28s
tests/beta	34s
tests/nightly	26s
tests/stable	31s
tests/1.64.0	31s
conclusion	0s
	18m 31s

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