docs: Update models/_docs/docs.md

Co-authored-by: Ty Hob / Brian Mesick <112640379+bmtcril@users.noreply.github.com>

models/_docs/docs.md

@@ -103,7 +103,7 @@ The datetime of the event sink run that published this block to ClickHouse. When
 {% enddocs %}
 
 {% docs username %}
-The username of the learner
+The username of the learner from the LMS user table. When Aspects PII syncing is off this will be empty.
 {% enddocs %}
 
 {% docs verb_id %}