[Dubbo-4331] Improve api doc (apache#5070)

Author: sonymoon

pom.xml

@@ -289,6 +289,7 @@
                                 <id>attach-javadoc</id>
                                 <goals>
                                     <goal>jar</goal>
+                                    <goal>aggregate</goal>
                                 </goals>
                                 <configuration>
                                     <additionalDependencies>
@@ -315,6 +316,11 @@
                                 <link>http://docs.oracle.com/javase/8/docs/api</link>
                             </links>
                             <doclint>none</doclint>
+                            <excludePackageNames>
+                                org.apache.dubbo.demo,org.apache.dubbo.demo.*
+                            </excludePackageNames>
+                            <doctitle>Apache Dubbo ${project.version} API</doctitle>
+                            <windowtitle>Apache Dubbo ${project.version} API</windowtitle>
                         </configuration>
                     </plugin>
                     <plugin>