Add necesarry step to enable DTrace (#4456)

- modify explanation for --enable dtrace option
 - add one step to set USE_ZEND_DTRACE env.

Author: egmc

features/dtrace.xml

@@ -91,10 +91,13 @@
     </informalexample>
    </para>
    <para>
-    This enables the static probes in core PHP.  Any PHP extensions
+    This makes the static probes available in core PHP.  Any PHP extensions
     that provide their own probes should be built separately as shared
     extensions.
    </para>
+   <para>
+    To enable probes, set <option>USE_ZEND_DTRACE=1</option> environment variable to the target PHP processes.
+   </para>
   </sect2>
 
   <sect2 xml:id="features.dtrace.static-probes">