docs(http1): say what the awaited writev size check guards (#179)

[EdmondDantes]

One comment. http_connection_send_strv_awaited compares transferred against its own
sum, which reads as a byte count check and is not one: all-or-nothing is libuv's write
contract, and on success the completion reports the sum the reactor was handed.

The review that produced this line also produced true-async/php-async#263, which fixes what
the comparison could not catch — a write silently narrowed to 32 bits and reported as
complete, and a request that outlived the handle it pointed at.

[github-actions]

Coverage

Total lines: 82.13% → 81.94% (-0.19 pp)

File	Baseline	Current	Δ	Touched
src/core/http_connection.c	75.39%	75.67%	+0.28 pp	●
src/http2/http2_session.c	88.68%	85.50%	-3.18 pp
src/http3/http3_listener.c	77.93%	77.33%	-0.60 pp
src/http3/http3_packet.c	85.22%	90.43%	+5.22 pp
src/http_body_stream.c	84.85%	75.76%	-9.09 pp
src/http_request.c	83.94%	80.83%	-3.11 pp

[EdmondDantes]

Folded into #191 — the three-line comment and the measurement it describes belong in one pull request, not two.