ref-manual: Re-write of the IMAGE_FEATURES variable description.

Added more explanatory text with appropriate links.  Included a
link to the new "how-to" section in the dev-manual.

(From yocto-docs rev: 929185c387d9f745857786086750bf68cb4c7b9b)

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

@@ -1209,11 +1209,19 @@ Core layer for images cannot be removed
             <glossdef>
                 <para>The list of features to include in an image.
                 Typically, you configure this variable in an image recipe.
-                Note that you can also add extra features to the image by using the
+                You can also configure it in the <filename>local.conf</filename>
+                file found in the 
+                <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>.
+                <note>You can also add extra features to the image by using the
                 <filename><link linkend='var-EXTRA_IMAGE_FEATURES'>EXTRA_IMAGE_FEATURES</link></filename> variable.
-                See the "<link linkend="ref-features">Features</link>" chapter for
+                </note>
-                information on feature support that ships with the Yocto
+                For a list of image features that ships with the Yocto Project,
-                Project.</para>
+                see the "<link linkend="ref-features-image">Images</link>" 
+                section.
+                For some examples that show how to customize your image by 
+                using this variable, see the 
+                "<ulink url='&YOCTO_DOCS_DEV_URL;#usingpoky-extend-customimage-imagefeatures'>Customizing Images Using Custom <filename>IMAGE_FEATURES</filename> and <filename>EXTRA_IMAGE_FEATURES</filename></ulink>"
+                section in the Yocto Project Development Manual.</para>
             </glossdef>
         </glossentry>