channel_id in message reference is optional

Author: msciotti

docs/resources/Channel.md

@@ -261,9 +261,11 @@ Represents a message sent in a channel within Discord.
 | Field       | Type      | Description                             |
 |-------------|-----------|-----------------------------------------|
 | message_id? | snowflake | id of the originating message           |
-| channel_id  | snowflake | id of the originating message's channel |
+| channel_id? \*  | snowflake | id of the originating message's channel |
 | guild_id?   | snowflake | id of the originating message's guild   |
 
+\* `channel_id` is optional when creating a reply, but will always be present when receiving an event/response that includes this data model.
+
 ###### Message Activity Types
 
 | Type         | Value |