docs/autostart: add swayidle with wlopm example

...to turn outputs on/off following the addition of
wlr-output-power-management protocol support (commit c23397f3)

docs/autostart

@@ -20,14 +20,24 @@ waybar >/dev/null 2>&1 &
 # as mako to function correctly. Thunderbird is an example of this.
 mako >/dev/null 2>&1 &
 
-# Lock screen after 5 minutes
+# Lock screen after 5 minutes; turn off display after another 5 minutes.
 #
-# Note that we do not yet support turning off displays in the context of idle
-# system power management. Disabling outputs, for example by
-# `wlr-randr --output whatever --off` re-arranges views (since a837fefc),
-# so is not a good idea.
-# A swayidle-friendly implementation may be written using
-# wlr-output-power-management-unstable-v1.xml and could then work with a client
-# such as https://git.sr.ht/~leon_plickat/wlopm
-swayidle -w timeout 300 'swaylock -f -c 000000' >/dev/null 2>&1 &
-
+# Note that in the context of idle system power management, it is *NOT* a good
+# idea to turn off displays by 'disabling outputs' for example by
+# `wlr-randr --output <whatever> --off` because this re-arranges views
+# (since a837fef). Instead use a wlr-output-power-management client such as
+# https://git.sr.ht/~leon_plickat/wlopm
+#
+# The swayidle example below only turns on/off one output. If you have
+# multiple outputs, you could write a script like this:
+#
+#       #!/bin/sh
+#       wlopm | while IFS=' ' read -r output state; do
+#               wlopm --toggle "${output}"
+#       done
+#
+swayidle -w \
+	timeout 3 'swaylock -f -c 000000' \
+	timeout 6 'wlopm --off eDP-1' \
+	resume 'wlopm --on eDP-1' \
+	before-sleep 'swaylock -f -c 000000' >/dev/null 2>&1 &