Document \ escaping of escapeshellarg on Windows

Cf. <https://bugs.php.net/69646>.

reference/exec/functions/escapeshellarg.xml

@@ -26,6 +26,8 @@
    On Windows, <function>escapeshellarg</function> instead replaces percent
    signs, exclamation marks (delayed variable substitution) and double quotes
    with spaces and adds double quotes around the string.
+   Furthermore, each streak of consecutive backslashes (<literal>\</literal>)
+   is escaped by one additional backslash.
   </para>
  </refsect1>