fix(compose): document what actually suppresses raw HTML, item 123

CMARK_OPT_SAFE has had no effect since cmark-gfm made safe mode the
default; the flag is retained for API compatibility and the real
protection is that CMARK_OPT_UNSAFE is never set. Measured against
0.29.0.gfm.13: rendering with OPT_DEFAULT alone, with OPT_SAFE, and with
OPT_UNSAFE shows the first two suppress a script element and a
javascript: link while the third leaks both.

The comment credited the flag, which would have sent the next reader to
the wrong place, and the test could not tell the two apart: it would
have passed just as well with the flag deleted. What it has to guard
against is OPT_UNSAFE being introduced, so it now also asserts that
unsafe links are stripped, which is a protection this gets for free and
previously asserted nothing about.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015muoUo2GdxmBDSp5vjYcbE

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