added examples to compile_id. thanks to Pete_M.

docs/en/programmers/api-variables/variable-compile-id.xml

@@ -4,9 +4,34 @@
  <title>$compile_id</title>
  <para>
   Persistant compile identifier. As an alternative to passing the same
-	   compile_id to each and every function call, you can set this compile_id
-	   and it will be used implicitly thereafter.
+  compile_id to each and every function call, you can set this
+  compile_id and it will be used implicitly thereafter.
  </para>
+ <para>
+  With a compile_id you can work around the limitation that you cannot
+  use the same compile_dir for different template_dirs.  If you set a
+  distinct compile_id for each template_dir then smarty can tell the
+  compiled templates apart by their compile_id.
+ </para>
+ <para>
+  If you have for example a prefilter that localizes your templates
+  (that is: translates language dependend parts) at compile time, then
+  you should use the current language as compile_id and you will get a
+  set of compiled templates for each language you use.
+ </para>
+ <para>
+  Another example would be to use the same compile directory across
+  multiple domains / multiple vhosts. eg:
+ </para>
+ <example>
+  <title>compile_id</title>
+  <programlisting role="php">
+<![CDATA[
+   $smarty->compile_id = $_SERVER['SERVER_NAME'];
+   $smarty->compile_dir = 'path/to/shared_compile_dir';
+]]>
+  </programlisting>
+ </example>
 </sect1>
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