ref-manual: Edits to the GRUB_GFXSERIAL variable.

Told where to set this variable.

(From yocto-docs rev: 6984f3ea58479e855762d0ab2e1d68f3e0759655)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-variables.xml

@@ -2159,7 +2159,10 @@
                 <para>
                     Configures the GNU GRand Unified Bootloader (GRUB) to have
                     graphics and serial in the boot menu.
-                    Set this variable to "1" to enable both in the menu.
+                    Set this variable to "1" in your
+                    <filename>local.conf</filename> or distribution
+                    configuration file to enable graphics and serial
+                    in the menu.
                 </para>
 
                 <para>