docs(discord): six documentation drifts vs actual behavior + `/voice join` missing from slash UI

[malaiwah]

Summary

During a deeper Discord audit pass I found the Discord integration docs disagree with the code in six places, and one tiny code bug: /voice join is advertised in docs but can't be picked from the slash-command autocomplete.

The six drifts

#	Where	Doc says	Code does
D1	neither website/docs/user-guide/messaging/discord.md nor environment-variables.md	DISCORD_PROXY is not mentioned	env var is consumed at gateway/platforms/discord.py:551 and two other call sites (image/audio/doc downloads), overriding HTTPS_PROXY / macOS scutil auto-detect
D2	nowhere	HERMES_DISCORD_VOICE_PACKET_DUMP is not mentioned	read at discord.py (VoiceReceiver init) with three accepted values off / errors (default) / all
D5	messaging/discord.md:119	Server Members Intent required	intents.members = any(not entry.isdigit() for entry in self._allowed_user_ids) — so only needed if DISCORD_ALLOWED_USERS contains usernames. All-numeric allowlists do not request it and the bot comes online fine with the Developer-Portal toggle OFF
D6	voice-mode.md:282	Server Members Intent maps voice SSRC to user IDs	SSRC→user_id mapping comes from the SPEAKING opcode (op 5) on the voice websocket (VoiceReceiver._install_speaking_hook in discord.py). Members intent plays no role here.
D7	voice-mode.md:333	/voice join is a slash command	the @discord.app_commands.choices list only contains channel — users cannot pick join from autocomplete. The runner at gateway/run.py happily accepts both, so typed /voice join works, but Discord's slash-UI gate rejects it.
D8	voice-mode.md:363	"bot automatically pauses its audio listener while playing TTS replies"	not pause — the bot switches to a raised-RMS-threshold barge-in mode after a guard window (VoiceReceiver.start_playback / _on_packet barge-in path). Users can interrupt mid-sentence. Tunables: voice.discord_vc.barge_in_guard, voice.discord_vc.barge_in_rms.

Fix

PR #NNN — docs corrections for all six, plus one line change: add join as an explicit Choice(name="join — join your voice channel", value="join") on the /voice slash command so the autocomplete matches the docs and the runner's dispatch.

Test

pytest tests/gateway/ -k discord -p asyncio → 193 passed, 1 skipped, 0 failed after the change.

No behavior change for existing callers — typed /voice channel / /voice join already worked via the runner; the only visible effect is the slash autocomplete now shows join alongside channel.