doc: clarify Cookie API usage #1639

Job	Run time
Check Style	6s
Docs	1m 39s
linux / nightly	1m 19s
Android	24s
MSRV	2m 32s
linux / stable	44s
unstable features	24s
linux / beta	45s
WASM	54s
macOS / stable	1m 40s
windows / stable-x86_64-msvc	3m 14s
windows / stable-i686-msvc	4m 13s
windows / stable-x86_64-gnu	8m 41s
windows / stable-i686-gnu	8m 52s
feat.: default-tls disabled	40s
feat.: rustls-tls	53s
feat.: rustls-tls-manual-roots	46s
feat.: native-tls	43s
feat.: default-tls and rustls-tls	58s
feat.: cookies	48s
feat.: blocking	57s
feat.: gzip	51s
feat.: brotli	54s
feat.: deflate	48s
feat.: json	53s
feat.: multipart	49s
feat.: stream	54s
feat.: socks/default-tls	48s
feat.: socks/rustls-tls	55s
feat.: trust-dns	1m 10s
CI is green	1s
	49m 15s

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