ref-manual: Added more detail to note for FILESEXTRAPATHS use.

Some discussion about this variable indicated that the importance
of the immediate expansion operator was not being called out
enough.  I added further information to the existing note that
states to be sure to use ":=" when using the variable.

(From yocto-docs rev: b97d3fa714a7c71356cd00548d3d01280034afc2)

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

@@ -1175,6 +1175,20 @@ Core layer for images cannot be removed
                     In the above example, the build system looks for files in
                     a directory that has the same name as the corresponding
                     append file.
+                    <note>
+                        When using the <filename>FILESEXTRAPATHS</filename>,
+                        be sure to use the immediate expansion
+                        (<filename>:=</filename>) operator and include
+                        the trailing separating colon character.
+                        Immediate expansion makes sure that BitBake evaluates
+                        <filename>THISDIR</filename> at the time the directive
+                        is encountered rather than at some later time when
+                        expansion might result in a directory that does not
+                        contain the files you need.
+                        The trailing colon character is necessary because you
+                        are directing BitBake to extend the path by prepending
+                        directories to the search path.
+                    </note>
                     Here is another common use:
                     <literallayout class='monospaced'>
      FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
@@ -1197,11 +1211,6 @@ Core layer for images cannot be removed
                     files, you allow multiple append files that reside in
                     different layers but are used for the same recipe to
                     correctly extend the path.
-                    <note>
-                        Be sure to use the immediate expansion
-                        (<filename>:=</filename>) operator and include 
-                        the trailing separating colon character.
-                    </note>
                 </para>
             </glossdef>
         </glossentry>