document 'set_empty'

--- a/src/Doc/Datatypes/Datatypes.thy	Mon May 12 17:42:54 2014 +0200
+++ b/src/Doc/Datatypes/Datatypes.thy	Wed May 14 11:37:48 2014 +0200
@@ -908,6 +908,9 @@
 \item[@{text "t."}\hthm{rel\_mono}\rm:] ~ \\
 @{thm list.rel_mono[no_vars]}
 
+\item[@{text "t."}\hthm{set\_empty}\rm:] ~ \\
+@{thm list.set_empty[no_vars]}
+
 \item[@{text "t."}\hthm{set\_map}\rm:] ~ \\
 @{thm list.set_map[no_vars]}