added $compile_id parameter

and a note about is_cached's internals

docs/en/programmers/api-functions/api-is-cached.xml

@@ -11,6 +11,7 @@
    <type>bool</type><methodname>is_cached</methodname>
    <methodparam><type>string</type><parameter>template</parameter></methodparam>
    <methodparam choice="opt"><type>string</type><parameter>cache_id</parameter></methodparam>
+   <methodparam choice="opt"><type>string</type><parameter>compile_id</parameter></methodparam>
   </methodsynopsis>
   <para>
    This returns &true; if there is a valid cache for this template.
@@ -37,6 +38,15 @@ $smarty->display("index.tpl");
    You can also pass a cache id as an optional second parameter
    in case you want multiple caches for the given template.
   </para>
+  <para>
+   You can supply a compile id as an optional third parameter.
+   If you omit that parameter the persistent 
+   <link linkend="variable.compile.id">$compile_id</link> is used.
+  </para>
+  <para>
+   If you do not want to pass a cache id but want to pass a compile
+   id you have to pass <literal>null</literal> as cache id.
+  </para>
   <example>
    <title>is_cached with multiple-cache template</title>
    <programlisting role="php">
@@ -53,6 +63,24 @@ $smarty->display("index.tpl", "FrontPage");
 ]]>
    </programlisting>
   </example>
+
+
+  <note>
+  <title>Technical Note</title>
+  <para>
+   If <literal>is_cached</literal> returns true it actually loads the
+   cached output and stores it internally. Any subsequent call to
+   <link linkend="api.display">display()</link> or
+   <link linkend="api.fetch">fetch()</link>
+   will return this internally stored output and does not try to reload
+   the cache file. This prevents a race condition that may occur when a
+   second process clears the cache between the calls to is_cached and to
+   display in the example above. This also means calls to
+   <link linkend="api.clear.cache">clear_cache()</link>
+   and other changes of the cache-settings may have no effect after
+   <literal>is_cached</literal> returned true.
+  </para>
+  </note>
  </refsect1>
 </refentry>
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