document property 'corec_code'

--- a/src/Doc/Datatypes/Datatypes.thy	Wed Jul 02 17:01:49 2014 +0200
+++ b/src/Doc/Datatypes/Datatypes.thy	Wed Jul 02 17:01:51 2014 +0200
@@ -1766,6 +1766,9 @@
 @{thm llist.corec(1)[no_vars]} \\
 @{thm llist.corec(2)[no_vars]}
 
+\item[@{text "t."}\hthm{corec\_code} @{text "[code]"}\rm:] ~ \\
+@{thm llist.corec_code[no_vars]}
+
 \item[@{text "t."}\hthm{disc\_corec}\rm:] ~ \\
 @{thm llist.disc_corec(1)[no_vars]} \\
 @{thm llist.disc_corec(2)[no_vars]}