Python adapter docs understate what shipped: trace mode, A11y, and the pytest config surface

[vishnuv688]

Part of #330. Successor to #295, which put the Python adapter on the site; this
is about the site now understating what the adapter does.

Since #295 closed (2026-08-25) the adapter gained trace mode (#330), the dense
filmstrip (#290), the A11y tree and element overlay (#341), per-action element
capture, viewport metadata, command selectors, and a pytest CLI/ini config
surface (#339, PR #355). None of that reached the docs.

Two of the stale claims tell a reader a shipped feature does not exist, which is
worse than being merely out of date.

website/docs/devtools/Selenium.md (Python tab), webdriverio/webdriverio

line	says	reality
359	"Trace mode is not available for the Python adapter yet - it streams to the live dashboard and does not write a trace.zip"	shipped; #330 closed
530	"Live mode only | No trace.zip is written yet, so Trace Mode, the offline player, and the dense filmstrip and A11y tab that come with them are not available."	all three shipped
258	DEVTOOLS_ENABLE=1 pytest tests/ as the only invocation	--devtools / --devtools-trace, or devtools / devtools_trace in [tool.pytest.ini_options]
331-335	"configured by environment variables rather than an options object"	CLI -> ini -> env precedence
429	"the plugin drives all of this from DEVTOOLS_ENABLE=1"	as above
463	"simply DEVTOOLS_ENABLE=1 in Python"	as above

This repo

README.md and packages/selenium-devtools-py/README.md carry the same gaps.
CONTRIBUTING requires both sides to move together, so they are one piece of work.

Acceptance criteria

The Python tab documents trace mode (--devtools-trace / devtools_trace,
where the archive lands, what it contains), the config precedence, and the A11y
tree / element overlay support. The "Live mode only" limitation row is gone. The
two READMEs say the same thing as the site.

Why it blocks #294

Publishing to PyPI while the public docs deny a shipped feature is worse than
not publishing: the first thing a new user reads would tell them trace mode is
unavailable.

Depends on #355 (the config surface it documents). Size: S.