docs: explain TLS backend features better #1649

Job	Run time
Check Style	17s
Docs	1m 31s
unstable features	38s
linux / nightly	1m 17s
Android	25s
MSRV	3m 33s
linux / stable	49s
WASM	47s
linux / beta	41s
macOS / stable	1m 49s
windows / stable-x86_64-msvc	3m 17s
windows / stable-i686-msvc	3m 10s
windows / stable-x86_64-gnu	9m 3s
windows / stable-i686-gnu	9m 36s
feat.: default-tls disabled	40s
feat.: rustls-tls	39s
feat.: rustls-tls-manual-roots	43s
feat.: native-tls	42s
feat.: default-tls and rustls-tls	46s
feat.: cookies	48s
feat.: blocking	1m 5s
feat.: gzip	53s
feat.: brotli	59s
feat.: deflate	49s
feat.: json	52s
feat.: multipart	45s
feat.: stream	51s
feat.: socks/default-tls	47s
feat.: socks/rustls-tls	55s
feat.: trust-dns	56s
CI is green	1s
	50m 4s

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