kernel-dev: Updated the in-tree metadata section

Fixes [YOCTO #5065]

I added a blurb at the end of the section that talks about in-tree
metadata.  The blurb addresses situations where the user has changed
the metadata but not updated the corresponding SRCREV variables
in the kernel receipe.

(From yocto-docs rev: 5d3dc0e6ebc40b937c3f3ef397e2c94e7cb3b69b)

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

documentation/kernel-dev/kernel-dev-advanced.xml

@@ -296,6 +296,20 @@
      $ git commit --allow-empty -m "Create orphan meta branch"
             </literallayout>
         </para>
+
+        <para>
+            If you modify the Metadata in the linux-yocto 
+            <filename>meta</filename> branch, you must not forget to update 
+            the 
+            <ulink url='&YOCTO_DOCS_REF_URL;#var-SRCREV'><filename>SRCREV</filename></ulink>
+            statements in the kernel's recipe.
+            In particular, you need to update the 
+            <filename>SRCREV_meta</filename> variable to match the commit in 
+            the <filename>KMETA</filename> branch you wish to use.
+            Changing the data in these branches and not updating the 
+            <filename>SRCREV</filename> statements to match will cause the 
+            build to fetch an older commit.
+        </para>
     </section>
 </section>