be more explicit on sourceReleaseAssemblyDescriptor property default value meaning and other available values

git-svn-id: https://svn.apache.org/repos/asf/maven/pom/trunk@1742049 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/asf/src/site-docs/apt/index.apt.vm b/asf/src/site-docs/apt/index.apt.vm
index 42e4f8b..60bc765 100644
--- a/asf/src/site-docs/apt/index.apt.vm
+++ b/asf/src/site-docs/apt/index.apt.vm
@@ -94,7 +94,8 @@
     is in the dependencies, and an execution is configured. The plugin
     is configured to take a <<<descriptorRef>>> name from the
     <<sourceReleaseAssemblyDescriptor>> property, which by default is
-    set to <<<source-release>>>.
+    set to <<<source-release>>> (zip only): you can override the property with
+    <<<source-release-zip-tar>>> (both zip and tar) or <<<source-release-tar>>> (tar only) values.
 
  * maven-deploy-plugin