explained how variables are made available.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@3864 71c3de6d-444a-0410-be80-ed276b4c234a

war/resources/help/matrix/axes.html

@@ -10,4 +10,8 @@
   which takes three values. When you configure it, Hudson will run 3 builds, each with
   different values assigned to the "database" variable to exhaustively cover
   the configuration matrix.
+  <p>
+  Variables specified here are made available as environment variables to the build.
+  In addition, for Ant and Maven, variables are also exposed as properties, as if
+  <tt>-D<i>variableName</i>=<i>value</i></tt> are specified on the command line.
 </div>
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