Add a description for USES=display

svn path=/head/; revision=43220

en_US.ISO8859-1/books/porters-handbook/uses.xml

@@ -109,6 +109,20 @@
     package install or removal.</entry>
 </row>
 
+<row>
+  <entry><literal>display</literal></entry>
+  <entry>(none), <literal>ARGS</literal></entry>
+
+  <entry>Set up a virtual display environment.  If the environment
+    variable <varname>DISPLAY</varname> is not set, then
+    <application>Xvfb</application> is added as a build dependency,
+    and <varname>CONFIGURE_ENV</varname> is extended with the port
+    number of the currently running instance of
+    <application>Xvfb</application>.  The <literal>ARGS</literal>
+    parameter defaults to <literal>install</literal> and controls the
+    phase around which to start and stop the virtual display.</entry>
+</row>
+
 <row>
   <entry><literal>dos2unix</literal></entry>
   <entry>(none)</entry>