Fix #69290: Invalid modes do not trigger errors

Author: cmb69

reference/exec/functions/proc-open.xml

@@ -95,6 +95,7 @@
          element is either <literal>r</literal> to pass the read end of the pipe
          to the process, or <literal>w</literal> to pass the write end) and
          <literal>file</literal> (the second element is a filename).
+         Note that anything else than <literal>w</literal> is treated like <literal>r</literal>.
         </member>
         <member>
          A stream resource representing a real file descriptor (e.g. opened file,