[Docs] Update samples to use MessageContent intent & update v2 => v3 guide (#2471)

docs/guides/v2_v3_guide/v2_to_v3_guide.md

@@ -37,6 +37,7 @@ _client = new DiscordSocketClient(config);
 - AllUnprivileged: This is a group of most common intents, that do NOT require any [developer portal] intents to be enabled.
   This includes intents that receive messages such as: `GatewayIntents.GuildMessages, GatewayIntents.DirectMessages`
 - GuildMembers: An intent disabled by default, as you need to enable it in the [developer portal].
+- MessageContent: An intent also disabled by default as you also need to enable it in the [developer portal].
 - GuildPresences: Also disabled by default, this intent together with `GuildMembers` are the only intents not included in `AllUnprivileged`.
 - All: All intents, it is ill advised to use this without care, as it _can_ cause a memory leak from presence.
   The library will give responsive warnings if you specify unnecessary intents.