Add ALT_DIGITS constant (#3473)

Co-authored-by: haszi <haszika80@gmail.com>

reference/strings/constants.xml

@@ -878,6 +878,17 @@
     </simpara>
    </listitem>
   </varlistentry>
+  <varlistentry xml:id="constant.alt-digits">
+   <term>
+    <constant>ALT_DIGITS</constant>
+    (<type>int</type>)
+   </term>
+   <listitem>
+    <simpara>
+     Alternative symbols for digits.
+    </simpara>
+   </listitem>
+  </varlistentry>
  </variablelist>
  <variablelist role="constant_list">
   <title><function>nl_langinfo</function> <constant>LC_MONETARY</constant> Category Constants</title>