docs(discord): document config.yaml-first mention settings

[szsunyuan]

What does this PR do?

Fixes the Discord docs to match what the adapter actually does for the two keys in #13685.

On current main, _discord_require_mention and _discord_free_response_channels read config.extra first and only fall back to DISCORD_* when the config key is unset. Fresh installs write discord.require_mention: true, so a conflicting .env value is ignored. configuration.md already says config.yaml wins for non-secret settings. The Discord page still said the opposite.

The issue offered either a code change (env wins) or a docs change (config wins). This PR takes the docs side: .env is for secrets, and changing those helpers to env-first would fight that design. Closed PR #13732 was the code-side attempt; it is not revived here.

Open PR #82150 rewrites global precedence toward env-wins and softens one Discord sentence to a link. It does not implement #13685 and leaves the exact “the env var wins” sentence the issue quotes. This PR is scoped to those two mention-gating keys only.

The mention-control comment “env vars win over config.yaml” for DISCORD_ALLOW_MENTION_* is left alone — that path still bridges YAML only when the env var is unset.

Related Issue

Fixes #13685

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/messaging/discord.md: Configuration Reference + Config File sections now say require_mention / free_response_channels are config-first
• zh-Hans user-guide/messaging/discord.md: same two sentences

How to Test

1. On main, confirm the stale lines: “Environment variables always take precedence” and “the env var wins.”
2. Confirm plugins/platforms/discord/adapter.py _discord_require_mention / _discord_free_response_channels still read config.extra first.
3. Confirm website/docs/user-guide/configuration.md still lists config.yaml above .env for non-secrets.
4. Confirm the allow_mentions snippet still says env vars win.

Docs-only; Python tests are not required. CI Docs Site runs lint:diagrams + build:fast.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass — N/A (docs-only; Python CI lane will not run)
• I've added tests for my changes — N/A (docs wording only)
• I've tested on my platform: macOS (darwin 25.5) — wording/grep against current main

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings)
• I've updated cli-config.yaml.example if I added/changed config keys — N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — N/A
• I've updated tool descriptions/schemas if I changed tool behavior — N/A

Screenshots / Logs

N/A — wording-only docs change.