docs(config): note that streaming-block changes need a gateway restart

[liuhao1024]

What does this PR do?

The Gateway Streaming section of the configuration guide tells users they can adjust display.platforms.<platform>.streaming toggles from the dashboard's Channels page or directly in ~/.hermes/config.yaml, but it never says when those changes take effect. In reality the two layers behave differently:

• The whole streaming: block (enabled, transport, edit_interval, buffer_threshold, cursor, fresh_final_after_seconds) is snapshotted once when the gateway runner is constructed (gateway/run.py, self.config = ... load_gateway_config_for_runner()), and every turn reads that snapshot (gateway/run_turn_runner.py, getattr(self._runner.config, 'streaming')). Editing those keys does nothing until the gateway is restarted.
• The per-platform display.platforms.<platform>.streaming value is resolved per message from a fresh load_user_config_effective() read of config.yaml, so the dashboard Channels toggles and direct config edits apply from the next message without a restart.
• A per-platform switch can only narrow the master switch (StreamingConfig.enabled_for: globally_enabled and (override is None or bool(override))), so flipping a platform toggle cannot turn streaming on until the master switch is enabled and the gateway has restarted.

This is exactly the trap #133804 hit: the docs describe enabling streaming but omit the restart requirement. This PR documents the actual behavior in the existing "Per-platform streaming defaults" note (option two of the issue's proposal — "document the restart requirement"; the behavior-change option would touch the gateway's config snapshot lifecycle, which is a much larger change). The zh-Hans translation mirrors the same addition.

Related Issue

Fixes #133804

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• website/docs/user-guide/configuration.md: extended the "Per-platform streaming defaults" note in the Gateway Streaming section with the restart requirement for the streaming: block, the no-restart behavior of the per-platform toggles, and the "per-platform can only narrow the master switch" rule
• website/i18n/zh-Hans/docusaurus-plugin-content-docs/current/user-guide/configuration.md: mirrored the same three facts in the zh-Hans translation of the note

How to Test

1. Code-path verification of the documented behavior (observed on current main):
   • gateway/run.py — the runner loads self.config once at construction (load_gateway_config_for_runner()), so streaming: keys are a startup snapshot.
   • gateway/run_turn_runner.py — each turn reads scfg = getattr(getattr(self._runner, 'config', None), 'streaming', None) (the snapshot) but resolves the per-platform value as ctx.resolve_display_setting(ctx.user_config, platform_key, "streaming"), where ctx.user_config comes from a per-message load_user_config_effective() read (_load_gateway_config() is uncached).
   • gateway/config.py — StreamingConfig.enabled_for() shows a per-platform value can only narrow globally_enabled.
2. python3 website/scripts/check_doc_links.py → Observed result: OK: no route-style links in hand-authored docs. (exit 0)
3. python3 -m pytest tests/website/ -q → Observed result: 46 passed in 1.74s
4. Markdown shape unchanged elsewhere: the diff is exactly two modified note lines (EN + zh-Hans), no new anchors or links introduced.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (docs(config): ...)
• I searched for existing PRs to make sure this isn't a duplicate (fix(telegram): coalesce native drafts and preserve growing text #133842 references [Bug]: Telegram streaming changes require an undocumented gateway restart #133804 as related but fixes [Bug]: Telegram native drafts burst updates and rewrite streamed prefixes #133841 and does not add the restart documentation)
• My PR contains only changes related to this fix/feature (no unrelated commits) — single commit touching two documentation files
• I've run pytest tests/ -q and all tests pass — N/A, docs-only change, no code touched; tests/website/ (the docs-related suite) passes: 46 passed
• I've added tests for my changes — N/A for a documentation update
• I've tested on my platform: macOS 26.4 (documentation verification only)

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — this PR is the documentation update
• I've updated cli-config.yaml.example if I added/changed config keys — N/A (no config keys added or changed)
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — N/A (docs only)
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A