Simplify features and make documentation call out feature gates #371

Job	Run time
cargo-careful	1m 49s
cross_test/s390x-unknown-linux-gnu/stable	6m 56s
clippy/stable	20s
blas-msrv	1m 0s
cross_test/i686-unknown-linux-gnu/stable	2m 16s
docs/nightly	21s
format/nightly	17s
miri	7m 56s
nostd/thumbv6m-none-eabi/stable	14s
tests/stable	3m 35s
tests/beta	3m 23s
tests/nightly	3m 43s
tests/1.64.0	6m 29s
conclusion	1s
	38m 20s

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