Documentation: Updated month and year date for 2.4.4 release.

Implemented the ENTITY method to apply the month and date
release time for the 2.4.4 release, which is now "December 2018".

Updated poky.ent to use the new ENTITY.

(From yocto-docs rev: 89f034c819288a936fd029dd754fcdd17a86ce43)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/kernel-dev/kernel-dev.xml

@@ -103,7 +103,7 @@
             </revision>
             <revision>
                 <revnumber>2.4.4</revnumber>
-                <date>November 2018</date>
+                <date>&REL_MONTH_YEAR;</date>
                 <revremark>Released with the Yocto Project 2.4.4 Release.</revremark>
             </revision>
         </revhistory>