Document know limitation about using references in conjunction with eager loading causing it to ignore custom select clauses.

[ci skip]

Fixes rails#24314

Author: vipulnsward

guides/source/active_record_querying.md

@@ -1251,8 +1251,9 @@ articles, all the articles would still be loaded. By using `joins` (an INNER
 JOIN), the join conditions **must** match, otherwise no records will be
 returned.
 
-
-
+NOTE: If an association is eager loaded as part of a join, any fields from a custom select clause will not present be on the loaded models. 
+This is because it is ambiguous whether they should appear on the parent record, or the child.
+ 
 Scopes
 ------