ref-manual: Adjusted IMAGE_FEATURES and EXTRA_IMAGE_FEATURES notes

Comments from Paul Eggleton applied.  Basically, reinforcing
enabling features from inside and outside of the image.  Changed
the wording of the respective notes.

(From yocto-docs rev: 23897c6ebc56dde63803293c0992b2d5c6ff7345)

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

@@ -954,10 +954,10 @@ Core layer for images cannot be removed
                     Although you can use this variable from within a recipe,
                     best practices dictate that you do not.
                     <note>
-                        Use the
+                        To enable primary features from within the image, use
+                        the
                         <link linkend='var-IMAGE_FEATURES'><filename>IMAGE_FEATURES</filename></link>
-                        variable from within a recipe to define the primary
+                        variable.
-                        list of features you want to add to the image.
                     </note>
                 </para>
 
@@ -1243,7 +1243,8 @@ Core layer for images cannot be removed
                     <ulink url='&YOCTO_DOCS_DEV_URL;#build-directory'>Build Directory</ulink>,
                     best practices dictate that you do not.
                     <note>
-                        Add extra features to the image by using the
+                        To enable extra features from outside the image,
+                        use the
                         <filename><link linkend='var-EXTRA_IMAGE_FEATURES'>EXTRA_IMAGE_FEATURES</link></filename> variable.
                     </note>
                     For a list of image features that ships with the Yocto