[PATCH] doc: drop some incorrect instructions

[liu.ming50@gmail.com]

From: Ming Liu <liu.ming50@gmail.com>

image_types bbclass now is being inherited mandatorily in image.bbclass
through the variable IMGCLASSES, and the users do not have to inherit
it in their customized image type bbclass, or put it in IMAGE_CLASSES.

Drop the incorrect descriptions, it's confusing the developers.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
---
 documentation/ref-manual/ref-classes.xml | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/documentation/ref-manual/ref-classes.xml b/documentation/ref-manual/ref-classes.xml
index 24d7a0a..876e242 100644
--- a/documentation/ref-manual/ref-classes.xml
+++ b/documentation/ref-manual/ref-classes.xml
@@ -1275,15 +1275,10 @@
     </para>
 
     <para>
-        By default, this class is enabled through the
-        <link linkend='var-IMAGE_CLASSES'><filename>IMAGE_CLASSES</filename></link>
-        variable in
+        By default, this class is enabled mandatorily in
         <link linkend='ref-classes-image'><filename>image.bbclass</filename></link>.
         If you define your own image types using a custom BitBake class and
-        then use <filename>IMAGE_CLASSES</filename> to enable it, the custom
-        class must either inherit <filename>image_types</filename> or
-        <filename>image_types</filename> must also appear in
-        <filename>IMAGE_CLASSES</filename>.
+        then use <filename>IMAGE_CLASSES</filename> to enable it.
     </para>
 
     <para>
-- 
2.7.4