detach process more reliably -- NB: viewer might be synchronous or asynchronous depending on settings;

--- a/src/Pure/System/doc.scala	Sun Jun 23 20:26:31 2013 +0200
+++ b/src/Pure/System/doc.scala	Sun Jun 23 21:15:42 2013 +0200
@@ -60,7 +60,8 @@
       } yield (dir, doc)
     docs match {
       case (dir, doc) :: _ =>
-        Isabelle_System.bash_env(dir.file, null, "\"$ISABELLE_TOOL\" display " + quote(doc) + " &")
+        Isabelle_System.bash_env(dir.file, null,
+          "\"$ISABELLE_TOOL\" display " + quote(doc) + " >/dev/null 2>/dev/null &")
       case Nil => error("Missing Isabelle documentation: " + quote(name))
     }
   }