ref-manual: Updated the DISTRO variable description

I added a cross-reference to the "DISTRO_NAME" variable as it is
the long version of the distribution name.

(From yocto-docs rev: bb5638409faadb9c6a5fbca729e4e6b8a220433d)

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

documentation/ref-manual/ref-variables.xml

@@ -3636,10 +3636,17 @@
                 <para role="glossdeffirst">
 <!--                <para role="glossdeffirst"><imagedata fileref="figures/define-generic.png" /> -->
                     The short name of the distribution.
-                    This variable corresponds to a distribution
-                    configuration file whose root name is the same as the
-                    variable's argument and whose filename extension is
-                    <filename>.conf</filename>.
+                    For information on the long name of the distribution, see
+                    the
+                    <link linkend='var-DISTRO_NAME'><filename>DISTRO_NAME</filename></link>
+                    variable.
+                </para>
+
+                <para>
+                    The <filename>DISTRO</filename> variable corresponds to a
+                    distribution configuration file whose root name is the
+                    same as the variable's argument and whose filename
+                    extension is <filename>.conf</filename>.
                     For example, the distribution configuration file for the
                     Poky distribution is named <filename>poky.conf</filename>
                     and resides in the
@@ -3664,9 +3671,9 @@
                     The value for <filename>DISTRO</filename> must not contain
                     spaces, and is typically all lower-case.
                     <note>
-                        If the <filename>DISTRO</filename> variable is blank, a set
-                        of default configurations are used, which are specified
-                        within
+                        If the <filename>DISTRO</filename> variable is blank,
+                        a set of default configurations are used, which are
+                        specified within
                         <filename>meta/conf/distro/defaultsetup.conf</filename>
                         also in the Source Directory.
                     </note>