documentation/poky-ref-manual/ref-variables.xml: update PREFERRED_VERSION

Added a more robust description and provided a couple of usage
examples.

(From yocto-docs rev: b8b842b57cc003f1351a551041fe4b3de2fcbfd6)

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

1 files changed, 12 insertions, 2 deletions

diff --git a/documentation/poky-ref-manual/ref-variables.xml b/documentation/poky-ref-manual/ref-variables.xml
index 777a91b81f..5d4e5b1ab3 100644
--- a/documentation/poky-ref-manual/ref-variables.xml
+++ b/documentation/poky-ref-manual/ref-variables.xml
@@ -842,8 +842,18 @@
                 <para>
                     If there are multiple versions of recipes available, this
                     variable determines which recipe should be given preference.
-                    The variable should be set to the <filename>$PV</filename> of the recipe 
-                    to whichy you want to give precedence.
+                    The variable must always be suffixed with the <filename>$PN</filename> 
+                    for which to select, and should be set to the 
+                    <filename>$PV</filename> to which you want to give precedence.
+                    You can use the "<filename>%</filename>" character as a wildcard
+                    to match any number of characters, which can be useful when 
+                    specifying versions that contain long revision number that could 
+                    potentially change.
+                    Here are two examples:
+                    <literallayout class='monospaced'>
+     PREFERRED_VERSION_python = "2.6.6"
+     PREFERRED_VERSION_linux-yocto = "3.0+git%" 
+                    </literallayout>
                 </para>
             </glossdef>
         </glossentry>