all: make 0 => unsafe { nil } an error, not a notice

doc/docs.md

@@ -6707,16 +6707,16 @@ struct Node {
 }
 
 // Reference fields must be initialized unless an initial value is declared.
-// Zero (0) is OK but use with caution, it's a nil pointer.
+// Nil is OK but use with caution, it's a nil pointer.
 foo := Node{
-	a: 0
+	a: unsafe { nil }
 }
 bar := Node{
 	a: &foo
 }
 baz := Node{
-	a: 0
-	b: 0
+	a: unsafe { nil }
+	b: unsafe { nil }
 }
 qux := Node{
 	a: &foo