runtime(doc): add reference to extendnew() at extend()

related: #16607

Signed-off-by: Christian Brabandt <[email protected]>

Author: chrisbra

runtime/doc/builtin.txt

@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2025 Feb 06
+*builtin.txt*	For Vim version 9.1.  Last change: 2025 Feb 17
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -3006,7 +3006,8 @@ extend({expr1}, {expr2} [, {expr3}])			*extend()*
 		When {expr3} is omitted then "force" is assumed.
 
 		{expr1} is changed when {expr2} is not empty.  If necessary
-		make a copy of {expr1} first.
+		make a copy of {expr1} first or use |extendnew()| to return a
+		new List/Dictionary.
 		{expr2} remains unchanged.
 		When {expr1} is locked and {expr2} is not empty the operation
 		fails.