docs: Remove #[doc(hidden)] and add documentation for internal traits #621

Job	Run time
cross_test/s390x-unknown-linux-gnu/stable	9m 58s
clippy/stable	31s
nostd/thumbv6m-none-eabi/stable	22s
Pass MSRV values to other jobs	4s
format/nightly	15s
miri	11m 11s
docs/nightly	20s
cargo-careful	2m 4s
cross_test/i686-unknown-linux-gnu/stable	2m 51s
tests/1.64.0	3m 29s
tests/nightly	3m 16s
tests/stable	2m 50s
blas-msrv	43s
tests/beta	2m 55s
conclusion	0s
	40m 49s

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