docs(bench): what a streamed chunk actually costs on HTTP/1 (#179)

[EdmondDantes]

#179 rests on "three submits and up to three suspensions per chunk". Measured on
e246dcf with the release PHP at ABI 0.25, strace -f -c around
tests/perf/servers/server_stream.php, two runs per chunk size differing only in the
number of chunks so the difference divides out startup:

chunk	chunks measured	writev	epoll_pwait	per chunk
4 KiB	960	960	1009	1.00 writev, 1.05 waits
64 KiB	480	480	510	1.00 writev, 1.06 waits

One submit and one park per chunk, flat in the chunk size — a 64 KiB frame takes the
copy-free path and still leaves as one writev, because its three pieces are three slots.

So the premise is gone. What a queue could still remove is the park, and only by making
the write fire-and-forget — which is exactly what isWritable() and tryWrite() would
then have nothing honest to say about.

[github-actions]

Coverage

Total lines: 82.13% → 82.20% (+0.07 pp)

File	Baseline	Current	Δ	Touched
src/core/http_connection.c	75.39%	75.67%	+0.28 pp	●
src/http3/http3_listener.c	77.93%	78.53%	+0.60 pp
src/http3/http3_packet.c	85.22%	90.43%	+5.22 pp
src/websocket/ws_session.c	89.67%	90.22%	+0.55 pp