Cygwin: add @cjksingle to release docs

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/release/3.1.4

@@ -16,6 +16,11 @@ What changed:
 - <signal.h>: New macro _NSIG replacing NSIG.  NSIG is now only
   visible to MISC builds, as on Linux.
 
+- The new locale modifier @cjksingle allows enforcing of single-width
+  character property for usually double-widthed characters.  This will
+  be supported by upcoming mintty releases.  For the reasoning, see
+  https://gitlab.freedesktop.org/terminal-wg/specifications/issues/9.
+
 
 Bug Fixes:
 ----------

winsup/doc/new-features.xml

@@ -75,6 +75,13 @@ set, or if an attempt is made to open a different type of socket, the
 errno is now EOPNOTSUPP instead of ENXIO.
 </para></listitem>
 
+<listitem><para>
+The new locale modifier @cjksingle allows enforcing of single-width
+character property for usually double-widthed characters.  This will
+be supported by upcoming mintty releases.  For the reasoning, see
+https://gitlab.freedesktop.org/terminal-wg/specifications/issues/9.
+</para></listitem>
+
 </itemizedlist>
 
 </sect2>