document property 'sel_set'

--- a/src/Doc/Datatypes/Datatypes.thy	Mon Jun 02 14:29:20 2014 +0200
+++ b/src/Doc/Datatypes/Datatypes.thy	Mon Jun 02 14:29:20 2014 +0200
@@ -861,6 +861,9 @@
 \item[@{text "t."}\hthm{set\_empty}\rm:] ~ \\
 @{thm list.set_empty[no_vars]}
 
+\item[@{text "t."}\hthm{sel\_set}\rm:] ~ \\
+@{thm list.sel_set[no_vars]}
+
 \item[@{text "t."}\hthm{map} @{text "[simp, code]"}\rm:] ~ \\
 @{thm list.map(1)[no_vars]} \\
 @{thm list.map(2)[no_vars]}