proc_close: clarify termination status in regard to proc_get_status

Explain how proc_get_status may interfere with proc_close return value.

Author: SinusPi

reference/exec/functions/proc-close.xml

@@ -48,6 +48,11 @@
    Returns the termination status of the process that was run. In case of 
    an error then <literal>-1</literal> is returned.
   </para>
+  <para>
+   If <function>proc_get_status</function> is called before <function>proc_close</function>,
+   and finds the process not in "running" state anymore, and retrieves the exit code, then
+   <function>proc_close</function> will also return <literal>-1</literal>.
+  </para>
   &note.sigchild;
  </refsect1>