Update Projection section in reference documentation.

For the time being we at least now document the current behaviour and how to cope with it.

See: #3286

Author: christophstrobl

src/main/antora/modules/ROOT/pages/repositories/projections.adoc

@@ -4,3 +4,28 @@
 include::{commons}@data-commons::page$repositories/projections.adoc[leveloffset=+1]
 
 NOTE: It is important to note that <<projections.dtos,Class-based projections>> with JPQL is limited to *constructor expressions* in your JPQL expression, e.g. `SELECT new com.example.NamesOnly(u.firstname, u.lastname) from User u`. (Note the usage of a FQDN for the DTO type!) This JPQL expression can be used in `@Query` annotations as well where you define any named queries. And it's important to point out that class-based projections do not work with native queries AT ALL. As a workaround you may use named queries with `ResultSetMapping` or the Hibernate specific https://docs.jboss.org/hibernate/orm/6.0/javadocs/org/hibernate/transform/ResultTransformer.html[`ResultTransformer`]
+
+[NOTE]
+====
+Some JPA providers may require additional hints when working with <<projections.dtos,Class-based projections>> especially when using those directly with method derived queries.
+If you are facing errors like `JpaSystemException: Specified result type did not match Query selection type` make sure to provide a constructor hint via `@PersistenceCreator` to be passed on, as outlined below:
+
+[source,java]
+----
+public class NamesOnly {
+
+    private final String firstname;
+    private final String lastname;
+
+    protected NamesOnly() { }
+
+    @PersistenceCreator
+    public NamesOnly(String firstname, String lastname) {
+        this.firstname = firstname;
+        this.lastname = lastname;
+    }
+
+    // ...
+}
+----
+====