docs/userguide: document behavior of tray_output with multiple bars

fixes #1855

diff --git a/docs/userguide b/docs/userguide
index eea7141b67b184d8a6bb3d9ef45f6823ff5f6fec..98e8dee748a3a49f7e4a3c4a003bda2aba11b76c 100644 (file)
--- a/docs/userguide
+++ b/docs/userguide
@@ -1371,6 +1371,11 @@ Note that you might not have a primary output configured yet. To do so, run:
 xrandr --output <output> --primary
 -------------------------
 
+Note that when you use multiple bar configuration blocks, either specify
+`tray_output primary` in all of them or explicitly specify `tray_output none`
+in bars which should not display the tray, otherwise the different instances
+might race each other in trying to display tray icons.
+
 === Tray padding
 
 The tray is shown on the right-hand side of the bar. By default, a padding of 2