Improve docs regarding state and extensions #4982

Job	Run time
cargo-hack	3m 7s
check-docs	22s
cargo-public-api-crates (axum)	32s
check	49s
cargo-public-api-crates (axum-core)	20s
cargo-deny check (advisories)	25s
cargo-public-api-crates (axum-extra)	29s
dependencies-are-sorted	14s
cargo-public-api-crates (axum-macros)	17s
cargo-deny check (bans licenses sources)	25s
Spell Check with Typos	6s
test-docs	1m 20s
test-msrv	1m 48s
test-nightly	1m 21s
armv5te-unknown-linux-musleabi	27s
test-versions (stable)	1m 22s
wasm32-unknown-unknown	21s
test-versions (beta)	1m 25s
	15m 10s

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