docs: explain the home passthru present in all JDKs and setting JAVA_HOME where required

doc/languages-frameworks/java.xml

@@ -61,7 +61,18 @@ installPhase =
   <literal>${jre}/bin/java</literal> instead of
   <literal>${jdk}/bin/java</literal>, you prevent your package from depending
   on the JDK at runtime.
- </para>
+</para>
+
+<para>
+  Note all JDKs passthru <literal>home</literal>, so if your application
+  requires environment variables like <envar>JAVA_HOME</envar> being set, that
+  can be done in a generic fashion with the <literal>--set</literal> argument
+  of <literal>makeWrapper</literal>:
+
+<programlisting>
+  --set JAVA_HOME ${jdk.home}
+</programlisting>
+</para>
 
  <para>
   It is possible to use a different Java compiler than <command>javac</command>