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/ga.yaml

@@ -347,6 +347,8 @@ gateway:
     label_off:             "As (téacs amháin)"
     label_voice_only:      "Ar (freagra gutha do theachtaireachtaí gutha)"
     label_all:             "TTS (freagra gutha do gach teachtaireacht)"
+    help:                  "{toggle}\n\n**Conas a oibríonn /voice**\n• `/voice on` — freagra gutha nuair a sheolann tú teachtaireacht gutha\n• `/voice tts` — freagra gutha do *gach* teachtaireacht\n• `/voice off` — ar ais go freagraí téacs amháin\n• `/voice status` — taispeáin an mód reatha\n• `/voice` (gan argóint) — scoránaigh go tapa idir air agus as{channels}"
+    help_channels:         "\n\n**Cainéil gutha bheo (Discord)**\n• Téigh isteach i gcainéal gutha ar dtús, ansin `/voice channel` — téim isteach, éistim agus labhraím mo fhreagraí\n• `/voice leave` — dícheangail ón gcainéal gutha"
 
   yolo:
     disabled:                   "⚠️ Mód YOLO **AS** don seisiún seo — beidh cead de dhíth d'orduithe contúirteacha."