docs: add cfg notes about http3 builder methods #1584

Job	Run time
Check Style	9s
Docs	1m 37s
unstable features	24s
linux / nightly	1m 19s
MSRV	1m 39s
linux / stable	49s
Android	25s
WASM	52s
linux / beta	1m 12s
macOS / stable	1m 40s
windows / stable-x86_64-msvc	4m 10s
windows / stable-i686-msvc	4m 33s
windows / stable-x86_64-gnu	7m 49s
windows / stable-i686-gnu	8m 32s
feat.: default-tls disabled	1m 3s
feat.: rustls-tls	53s
feat.: rustls-tls-manual-roots	49s
feat.: native-tls	53s
feat.: default-tls and rustls-tls	56s
feat.: cookies	1m 13s
feat.: blocking	1m 3s
feat.: gzip	1m 3s
feat.: brotli	56s
feat.: deflate	1m 17s
feat.: json	1m 7s
feat.: multipart	53s
feat.: stream	56s
feat.: socks/default-tls	1m 12s
feat.: socks/rustls-tls	57s
feat.: trust-dns	1m 5s
CI is green	1s
	51m 27s

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