[incubator-kie-drools#5752] [docs] Add explanation for DRL declared R…

…ule Unit (#5755)

drools-docs/src/modules/ROOT/pages/language-reference/_drl-rules.adoc

@@ -144,6 +144,11 @@ rule "Using a rule unit with a declared type"
 end
 ----
 
+[NOTE]
+====
+If you declare a rule unit in DRL, the rule unit class will be generated by {PRODUCT} at build time. It means that you can't refer to the class in your client codes. Declared rule units are only useful when the client is also auto-generated, for example, REST endpoint generation by Drools-Quarkus integration.
+====
+
 To separate the fact types from the rule unit for use with other DRL rules, you can declare the types in a separate DRL file and then use the DRL rule file to declare the data sources by using the `RuleUnitData` interface implementation:
 
 .Example DRL type declaration as a separate file