Add missing description for focus_on_window_activation command in docs.

sway/sway.5.scd

@@ -536,6 +536,13 @@ The default colors are:
 	set to _always_, the window under the cursor will always be focused, even
 	after switching between workspaces.
 
+*focus_on_window_activation* smart|urgent|focus|none
+	This option determines what to do when an xwayland client requests
+	window activation. If set to _urgent_, the urgent state will be set
+	for that window. If set to _focus_, the window will become focused.
+	If set to _smart_, the window will become focused only if it is already
+	visible, otherwise the urgent state will be set. Default is _smart_.
+
 *focus_wrapping* yes|no|force
 	This option determines what to do when attempting to focus over the edge
 	of a container. If set to _no_, the focused container will retain focus,