Fix portal section in the docs. (#8048)

* Update portal sections (outdated message).

documentation/portal/portal-eclipse.asciidoc

@@ -7,6 +7,8 @@ layout: page
 [[portal.eclipse]]
 = Creating a Generic Portlet in Eclipse
 
+*_This section has not yet been updated for Vaadin Framework 8._*
+
 Here we describe the creation of a generic portlet project in Eclipse. You can
 use the Maven archetypes also in other IDEs or without an IDE.
 

documentation/portal/portal-liferay.asciidoc

@@ -7,6 +7,8 @@ layout: page
 [[portal.liferay]]
 = Developing Vaadin Portlets for Liferay
 
+*_This section has not yet been updated for Vaadin Framework 8._*
+
 A Vaadin portlet requires resources such as the server-side Vaadin libraries, a
 theme, and a widget set. You have two basic ways to deploy these: either
 globally in Liferay, so that the resources are shared between all Vaadin