docs: explain TLS backend features better #1648

Job	Run time
Docs	1m 31s
Check Style	4s
Android	24s
MSRV	4m 26s
linux / nightly	1m 17s
linux / stable	49s
unstable features	24s
linux / beta	57s
WASM	46s
macOS / stable	2m 41s
windows / stable-x86_64-msvc	4m 35s
windows / stable-i686-msvc	3m 54s
windows / stable-x86_64-gnu	9m 36s
windows / stable-i686-gnu	8m 25s
feat.: default-tls disabled	49s
feat.: rustls-tls	53s
feat.: rustls-tls-manual-roots	1m 7s
feat.: native-tls	49s
feat.: default-tls and rustls-tls	56s
feat.: cookies	57s
feat.: blocking	1m 3s
feat.: gzip	53s
feat.: brotli	56s
feat.: deflate	55s
feat.: json	48s
feat.: multipart	51s
feat.: stream	54s
feat.: socks/default-tls	51s
feat.: socks/rustls-tls	58s
feat.: trust-dns	1m 8s
CI is green	1s
	54m 38s

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