clarify request_parse_body behavior with consumed body

Author: jordikroon

reference/network/functions/request-parse-body.xml

@@ -38,7 +38,8 @@
   <caution>
    <simpara>
     <function>request_parse_body</function> consumes the request body without
-    buffering it to the <literal>php://input</literal> stream.
+    buffering it to the <literal>php://input</literal> stream. If the body was 
+    already consumed earlier, the raw input stream may no longer be available.
    </simpara>
   </caution>
  </refsect1>