Merge pull request #4262 from segmentio/tanjinhong72-patch-1

Added information on recording userId for mailchimp - not to use trait.id

Author: stayseesong

src/connections/destinations/catalog/mailchimp/index.md

@@ -90,7 +90,7 @@ Once Mailchimp has processed the new subscriber you'll see it show up in your li
 
 ### Recording userId
 
-To record a Segment `userId` in Mailchimp, you must pass the userID as a trait on your `identify()` calls. Segment doesn't automatically map the userID to any Mailchimp properties.
+To record a Segment `userId` in Mailchimp, pass the user ID as a trait on your `identify()` calls. Don't pass the `userId` as a trait ID because the  `trait.id` is a reserved trait. Instead, pass the `userId` in a trait name with the corresponding merge field in Mailchimp. Segment doesn't automatically map the user ID to a Mailchimp property.
 
 ### Overriding List ID (Also now referred to as Audience ID)