Updated runtime files.

runtime/doc/eval.txt

@@ -1,4 +1,4 @@
-*eval.txt*	For Vim version 7.4.  Last change: 2015 Apr 11
+*eval.txt*	For Vim version 7.4.  Last change: 2015 Apr 30
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2961,6 +2961,8 @@ extend({expr1}, {expr2} [, {expr3}])			*extend()*
 		{expr1} is changed when {expr2} is not empty.  If necessary
 		make a copy of {expr1} first.
 		{expr2} remains unchanged.
+		When {expr1} is locked and {expr2} is not empty the operation
+		fails.
 		Returns {expr1}.