docs(browser): document local startup timeout bounds (#71672)

* docs(browser): document local startup timeout bounds

The new browser.localLaunchTimeoutMs and browser.localCdpReadyTimeoutMs
options are clamped to MAX_BROWSER_STARTUP_TIMEOUT_MS (120000 ms) by
normalizeStartupTimeoutMs in extensions/browser/src/browser/config.ts,
and zero/negative/non-finite values fall back to the defaults. Without
this in the configuration reference, users setting a higher value see
no error and silently get the 120 s ceiling, or set 0 expecting 'no
timeout' and silently get the default.

* docs(browser): clarify startup timeout validation

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

docs/tools/browser.md

@@ -200,7 +200,8 @@ Browser settings live in `~/.openclaw/openclaw.json`.
   process to expose its CDP HTTP endpoint. `localCdpReadyTimeoutMs` is the
   follow-up budget for CDP websocket readiness after the process is discovered.
   Raise these on Raspberry Pi, low-end VPS, or older hardware where Chromium
-  starts slowly. Values are capped at 120000 ms.
+  starts slowly. Values must be positive integers up to `120000` ms; invalid
+  config values are rejected.
 - `actionTimeoutMs` is the default budget for browser `act` requests when the caller does not pass `timeoutMs`. The client transport adds a small slack window so long waits can finish instead of timing out at the HTTP boundary.
 - `tabCleanup` is best-effort cleanup for tabs opened by primary-agent browser sessions. Subagent, cron, and ACP lifecycle cleanup still closes their explicit tracked tabs at session end; primary sessions keep active tabs reusable, then close idle or excess tracked tabs in the background.