patch 8.2.1698: cannot lock a variable in legacy Vim script like in Vim9

Problem:    Cannot lock a variable in legacy Vim script like in Vim9.
Solution:   Make ":lockvar 0" work.

runtime/doc/eval.txt

@@ -12364,7 +12364,9 @@ text...
 <			is equivalent to: >
 				:let x = 1
 				:lockvar! x
-<			This is useful if you want to make sure the variable
+<			NOTE: in Vim9 script `:const` works differently, see
+			|vim9-const|
+			This is useful if you want to make sure the variable
 			is not modified.  If the value is a List or Dictionary
 			literal then the items also cannot be changed: >
 				const ll = [1, 2, 3]
@@ -12404,6 +12406,8 @@ text...
 
 			[depth] is relevant when locking a |List| or
 			|Dictionary|.  It specifies how deep the locking goes:
+				0	Lock the variable {name} but not its
+					value.
 				1	Lock the |List| or |Dictionary| itself,
 					cannot add or remove items, but can
 					still change their values.
@@ -12417,7 +12421,14 @@ text...
 					|Dictionary|, one level deeper.
 			The default [depth] is 2, thus when {name} is a |List|
 			or |Dictionary| the values cannot be changed.
-								*E743*
+
+			Example with [depth] 0: >
+				let mylist = [1, 2, 3]
+				lockvar 0 mylist
+				let mylist[0] = 77   	" OK
+				call add(mylist, 4]  	" OK
+				let mylist = [7, 8, 9]  " Error!
+<								*E743*
 			For unlimited depth use [!] and omit [depth].
 			However, there is a maximum depth of 100 to catch
 			loops.