Updated docs regarding constraints of Handler arguments #4192

Job	Run time
check	2m 16s
cargo-public-api-crates (axum)	58s
check-docs	24s
cargo-hack	5m 50s
cargo-public-api-crates (axum-core)	55s
cargo-public-api-crates (axum-extra)	56s
cargo-deny check (advisories)	27s
cargo-public-api-crates (axum-macros)	52s
dependencies-are-sorted	42s
cargo-deny check (bans licenses sources)	35s
Spell Check with Typos	4s
test-docs	1m 58s
test-msrv	4m 34s
armv5te-unknown-linux-musleabi	48s
test-nightly	2m 12s
test-versions (stable)	3m 46s
test-versions (beta)	4m 40s
wasm32-unknown-unknown	39s
	32m 36s

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