Fix incorrect apidoc for 'kotlin'

KDoc was confused because now there are two package fragments for package
'kotlin' (built-ins and stdlib). KDoc's KPackage now has a list of descriptors,
not only one.

This fixes missing documentation for top-level members in package 'kotlin' in
the apidoc

 #KT-2521 Fixed

libraries/docs/apidoc/pom.xml

@@ -40,6 +40,8 @@
                 <version>${project.version}</version>
                 <configuration>
                     <sources>
+                        <source>${basedir}/../../../core/builtins/native</source>
+                        <source>${basedir}/../../../core/builtins/src</source>
                         <source>${basedir}/../../stdlib/src</source>
                         <source>${basedir}/../../kunit/src/main/kotlin</source>
                         <source>${basedir}/../../kotlin-jdbc/src/main/kotlin</source>