ref-manual: Edits to PREFERRED_PROVIDER variable.

(From yocto-docs rev: 1875121738e199ce99739daadf9a95f61da36a42)

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

@@ -2659,10 +2659,10 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
                 <para>
                     If multiple recipes provide an item, this variable
                     determines which recipe should be given preference.
-                    The variable must always be suffixed with the name of the
-                    provided item, and should be set to the
-                    <filename>PN</filename> of the recipe
-                    to which you want to give precedence.
+                    You should always suffix the variable with the name of the
+                    provided item, and you should set it to the
+                    <link linkend='var-PN'><filename>PN</filename></link>
+                    of the recipe to which you want to give precedence.
                     Here is an example:
                     <literallayout class='monospaced'>
      PREFERRED_PROVIDER_virtual/xserver = "xserver-xf86"