ref-manual: Added TUNEVALID[feature] variable to glossary.

Fixes [YOCTO #5463]

(From yocto-docs rev: e3e69cbe36e0097fdb19ccde2b447c42a59b486f)

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

@@ -9042,6 +9042,25 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
             </glossdef>
         </glossentry>
 
+        <glossentry id='var-TUNEVALID'><glossterm>TUNEVALID[&lt;feature&gt;]</glossterm>
+            <glossdef>
+                <para>
+                    Specifies a valid CPU or Application Binary Interface (ABI)
+                    tuning feature.
+                    The specified feature is stored as a flag.
+                    Valid features are specified in the machine include files
+                    (e.g. <filename>meta/conf/machine/include/arm/arch-arm.inc</filename>).
+                    Here is an example from that file:
+                    <literallayout class='monospaced'>
+     TUNEVALID[bigendian] = "Enable big-endian mode."
+                    </literallayout>
+                    See the machine include files in the
+                    <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>
+                    for these features.
+                </para>
+            </glossdef>
+        </glossentry>
+
     </glossdiv>
 
     <glossdiv id='var-glossary-u'><title>U</title>