sway.1.scd: document environment vars set by sway

This just documents the few environment variables set by sway in
sway.1.scd

sway/sway.1.scd

@@ -80,6 +80,27 @@ _XKB\_DEFAULT\_VARIANT_, _XKB\_DEFAULT\_OPTIONS_
 	preferred way to configure the keyboard is via the configuration file, see
 	*sway-input*(5).
 
+The following environment variables are set by sway:
+
+_DISPLAY_
+	If compiled with Xwayland support and Xwayland is not disabled by the
+	config, this will be set to the name of the X display used for Xwayland
+
+_I3SOCK_
+	For compatibility with i3, specifies the path to the sway IPC socket
+
+_SWAYSOCK_
+	Specifies the path to the sway IPC socket
+
+_WAYLAND_DISPLAY_
+	Specifies the name of the Wayland display that sway is running on
+
+_XCURSOR_SIZE_
+	Specifies the configured cursor size
+
+_XCURSOR_THEME_
+	Specifies the configured cursor theme
+
 # AUTHORS
 
 Maintained by Drew DeVault <sir@cmpwn.com>, who is assisted by other open