docs: add cfg notes about http3 builder methods (#2070) #1585

Job	Run time
Docs	1m 33s
Check Style	5s
Android	28s
linux / nightly	1m 17s
MSRV	1m 43s
linux / stable	57s
unstable features	24s
WASM	1m 22s
linux / beta	1m 12s
macOS / stable	1m 38s
windows / stable-x86_64-msvc	3m 58s
windows / stable-i686-msvc	3m 56s
windows / stable-x86_64-gnu	7m 42s
windows / stable-i686-gnu	7m 43s
feat.: default-tls disabled	46s
feat.: rustls-tls	47s
feat.: rustls-tls-manual-roots	49s
feat.: native-tls	51s
feat.: default-tls and rustls-tls	57s
feat.: cookies	57s
feat.: blocking	1m 3s
feat.: gzip	54s
feat.: brotli	59s
feat.: deflate	55s
feat.: json	58s
feat.: multipart	59s
feat.: stream	58s
feat.: socks/default-tls	49s
feat.: socks/rustls-tls	1m 9s
feat.: trust-dns	1m 13s
CI is green	1s
	49m 3s

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