GAclientId may cause user merge (#4651)

* Update externalids.md

* Update externalids.md

Author: Panandhan22

src/unify/identity-resolution/externalids.md

@@ -34,6 +34,9 @@ Segment automatically promotes the following traits and IDs in track and identif
 | ios.idfa           | context.device.advertisingId when context.device.type = 'ios'     |
 | ios.push_token     | context.device.token when context.device.type = 'ios'                                                         |
 
+> note ""
+> The Google clientID(ga_clientid) is a unique value created for each browser-device pair and will exist for 2 years if the cookie is not cleared. The analytics.reset() call should be triggered from Segment end when the user logs off. This call will clear the cookies and local Storage created by Segment. It doesn’t clear data from other integrated tools. So on the next login, the user will be assigned with a new unique anonymous_id, but the same ga_clientid will remain if this cookie is not cleared. Hence, the profiles with different anonymous_id but with same ga_clientid will get merged.
+
 ## Custom externalIDs
 
 Unify resolves identity for any other externalIDs that you bind to users - such as a phone number or any custom identifier that you support.