xdg-shell: Improve error enum formatting some

The long lines stood out, break them by putting the summary on its own
line.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Mike Blumenkrantz <zmike@osg.samsung.com>
Reviewed-by: Benoit Gschwind <gschwind@gnu-log.net>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

unstable/xdg-shell/xdg-shell-unstable-v6.xml

@@ -37,10 +37,14 @@
 
     <enum name="error">
       <entry name="role" value="0" summary="given wl_surface has another role"/>
-      <entry name="defunct_surfaces" value="1" summary="xdg_shell was destroyed before children"/>
-      <entry name="not_the_topmost_popup" value="2" summary="the client tried to map or destroy a non-topmost popup"/>
-      <entry name="invalid_popup_parent" value="3" summary="the client specified an invalid popup parent surface"/>
-      <entry name="invalid_surface_state" value="4" summary="the client provided an invalid surface state"/>
+      <entry name="defunct_surfaces" value="1"
+	     summary="xdg_shell was destroyed before children"/>
+      <entry name="not_the_topmost_popup" value="2"
+	     summary="the client tried to map or destroy a non-topmost popup"/>
+      <entry name="invalid_popup_parent" value="3"
+	     summary="the client specified an invalid popup parent surface"/>
+      <entry name="invalid_surface_state" value="4"
+	     summary="the client provided an invalid surface state"/>
     </enum>
 
     <request name="destroy" type="destructor">