clipboardSync: off by default

It is built from source, which makes the first switch long. The template
lists it as a commented option; status shows clipboard-sync only while it
runs or its autostart link exists.

docs/session.md

@@ -132,7 +132,8 @@ steamFrame.keyboard = { layout = "de"; variant = "nodeadkeys"; };
 
 ## Clipboard sync
 
-`clipboardSync.enable`, module `clipboard-sync`. On by default.
+`clipboardSync.enable`, module `clipboard-sync`. Off by default: it is
+built from source, so the first switch with it takes a while.
 
 **Problem:** the Steam session's X displays and the nested desktop have
 separate clipboards.