update doc example to not use `isBarrierGuard`

docs/codeql/codeql-language-guides/using-flow-labels-for-precise-data-flow-analysis.rst

@@ -139,29 +139,24 @@ is a barrier guard blocking flow through the use of ``data`` on the right-hand s
 At this point we know that ``data`` has evaluated to a truthy value, so it cannot be ``null``
 anymore.
 
-Implementing this additional condition is easy. We implement a subclass of ``DataFlow::BarrierGuardNode``:
+Implementing this additional condition is easy. We implement a predicate with the following signature:
 
 .. code-block:: ql
 
-  class TruthinessCheck extends DataFlow::BarrierGuardNode, DataFlow::ValueNode {
-    SsaVariable v;
-
-    TruthinessCheck() {
-      astNode = v.getAUse()
-    }
-
-    override predicate blocks(boolean outcome, Expr e) {
-      outcome = true and
-      e = astNode
-    }
+  private predicate truthinessCheck(DataFlow::GuardNode g, ControlFlowNode node, boolean branch) {
+    exists(SsaVariable v |
+      g = v.getAUse() and
+      node = g and
+      branch = true
+    )
   }
 
-and then use it to override predicate ``isBarrierGuard`` in our configuration class:
+and then use it to override predicate ``isBarrier`` in our configuration class:
 
 .. code-block:: ql
 
-  override predicate isBarrierGuard(DataFlow::BarrierGuardNode guard) {
-    guard instanceof TruthinessCheck
+  override predicate isBarrier(DataFlow::Node node) {
+    node = DataFlow::BarrierGuard<truthinessCheck/3>::getABarrierNode()
   }
 
 With this change, we now flag the problematic case and don't flag the unproblematic case above.