protocol: improve fullscreen_requested docs

 protocol/river-window-management-v1.xml | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/protocol/river-window-management-v1.xml b/protocol/river-window-management-v1.xml
index dd9e556..608225d 100644
--- a/protocol/river-window-management-v1.xml
+++ b/protocol/river-window-management-v1.xml
@@ -864,7 +864,10 @@
     <event name="fullscreen_requested">
       <description summary="the window requested to be fullscreen">
         The xdg-shell protocol for example allows windows to request that they
-        be made fullscreen and allows them to provide an output preference.
+        be made fullscreen and allows them to provide an optional output hint.
+
+        If the output argument is null, the window has no preference and the
+        window manager should choose an output.
 
         The window manager is free to honor this request using
         river_window_v1.fullscreen or ignore it.