Add note to shell_exec that the backtick operator is deprecated (php#5719)

* Add note to shell_exec that the backtick operator is deprecated

See php#5017

---------

Co-authored-by: Jordi Kroon <jordikroon@php.net>

Author: cookieguru

reference/exec/functions/shell-exec.xml

@@ -13,9 +13,9 @@
    <type class="union"><type>string</type><type>false</type><type>null</type></type><methodname>shell_exec</methodname>
    <methodparam><type>string</type><parameter>command</parameter></methodparam>
   </methodsynopsis>
-  <para>
-   This function is identical to the <link linkend="language.operators.execution">backtick operator</link>.
-  </para>
+  <simpara>
+   This function is identical to the deprecated <link linkend="language.operators.execution">backtick operator</link>.
+  </simpara>
   <note>
    <para>
     On Windows, the underlying pipe is opened in text mode which can cause the