request_parse_body: Clarify behavior when body already consumed (#5504)

Fixes #5270

Author: lacatoire

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

@@ -37,8 +37,12 @@
 
   <caution>
    <simpara>
+    The request body can only be consumed once.
     <function>request_parse_body</function> consumes the request body without
     buffering it to the <literal>php://input</literal> stream.
+    Conversely, if the body has already been read (e.g. via
+    <literal>php://input</literal>), <function>request_parse_body</function>
+    will return empty data.
    </simpara>
   </caution>
  </refsect1>