IBX-9107 Added more details about user

Author: nkostiv

docs/personalization/tracking_with_ibexa-tracker.md

@@ -22,8 +22,10 @@ It activates the tracking by inserting <https://cdn.perso.ibexa.co/ibexa-track
 <https://cdn.perso.ibexa.co/ibexa-tracker.min.js> into the page.
 
 To use this mechanism on your pages, copy the code snippet below, and replace 
-`<YOUR_MANDATOR_ID>` with the customer ID, `<PAGE_ID>` with the page/item identifier 
-and `<USER_ID>` with the one generated by your user identifier. 
+`<YOUR_MANDATOR_ID>` with the customer ID, `<PAGE_ID>` with the page/item identifier .
+`<USER_ID>`  - empty string for cookie based anonymous user or replaced with the value generated
+by your user identifier system for logged in user.
+
 All identifiers can be any form of string. 
 Paste this snippet into your website template page so that it appears before 
 the closing `</head>` tag.