Add documentation about Karaf OSGi services available

git-svn-id: https://svn.apache.org/repos/asf/karaf/trunk@1546555 13f79535-47bb-0310-9956-ffa450edef68
[KARAF-2511] Review the Service Wrapper page of the documentation

git-svn-id: https://svn.apache.org/repos/asf/karaf/trunk@1546556 13f79535-47bb-0310-9956-ffa450edef68