Slightly improve serde documentation #3236

Job	Run time
rust_msrv (ubuntu-latest)	1m 17s
features_check (ubuntu-latest)	5m 14s
rust_versions (ubuntu-latest, stable)	1m 7s
rust_versions (ubuntu-latest, beta)	1m 19s
alternative_targets (ubuntu-latest, wasm32-unknown-emscripten)	14s
rust_versions (ubuntu-latest, nightly)	1m 8s
timezones (ubuntu-latest, ACST-9:30)	1m 23s
alternative_targets (ubuntu-latest, aarch64-apple-ios)	20s
timezones (ubuntu-latest, EST4)	1m 38s
alternative_targets (ubuntu-latest, aarch64-linux-android)	18s
timezones (ubuntu-latest, UTC0)	1m 19s
check-docs	20s
timezones (ubuntu-latest, Asia/Katmandu)	1m 34s
timezones (macos-latest, ACST-9:30)	1m 4s
cross-targets (x86_64-unknown-illumos)	1m 10s
timezones (macos-latest, EST4)	1m 12s
cross-tests (ubuntu-latest)	2m 19s
timezones (macos-latest, UTC0)	56s
timezones (macos-latest, Asia/Katmandu)	1m 1s
no_std (ubuntu-latest, thumbv6m-none-eabi)	13s
test_wasi (ubuntu-latest, wasm32-wasi)	28s
timezones (windows-latest, ACST-9:30)	2m 31s
no_std (ubuntu-latest, x86_64-fortanix-unknown-sgx)	9s
timezones (windows-latest, EST4)	1m 55s
timezones (windows-latest, UTC0)	2m 38s
test_wasm (ubuntu-latest)	24s
timezones (windows-latest, Asia/Katmandu)	1m 45s
	34m 56s

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