feat(gateway): explain /voice usage when toggled bare (#39766)

A bare /voice silently toggled on/off with a one-line result, leaving
users with no idea what the modes mean or that Discord also supports
TTS-all and live voice-channel join/leave. Bare /voice now still
toggles but appends a usage explainer covering on/off/tts/status, with
the Discord voice-channel lines shown only on adapters that support
them.

Adds gateway.voice.help + gateway.voice.help_channels across all 16
locales (placeholders {toggle}/{channels}).

locales/zh.yaml

@@ -343,6 +343,8 @@ gateway:
     label_off:             "关闭（仅文本）"
     label_voice_only:      "开启（仅对语音消息进行语音回复）"
     label_all:             "TTS（对所有消息进行语音回复）"
+    help:                  "{toggle}\n\n**/voice 用法**\n• `/voice on` — 当你发送语音消息时用语音回复\n• `/voice tts` — 对*每条*消息都用语音回复\n• `/voice off` — 恢复为纯文本回复\n• `/voice status` — 显示当前模式\n• `/voice`（无参数）— 在开启和关闭之间快速切换{channels}"
+    help_channels:         "\n\n**实时语音频道 (Discord)**\n• 先加入一个语音频道，然后 `/voice channel` — 我会加入、聆听并用语音回复\n• `/voice leave` — 断开语音频道"
 
   yolo:
     disabled:                   "⚠️ 本会话 YOLO 模式 **已关闭** — 危险命令将需要批准。"