Expand documentation of hidden nodes

Author: owen-mc

docs/ql-libraries/dataflow/dataflow.md

@@ -618,12 +618,21 @@ impact on performance for large databases.
 
 ### Hidden nodes
 
-Certain synthetic nodes can be hidden to exclude them from occurring in path
-explanations. This is done through the following predicate:
+Certain synthetic nodes are hidden by default to prevent them from occurring in
+path explanations. Which nodes are included in this is determined by the
+following predicate:
+
 ```ql
 predicate nodeIsHidden(Node n)
 ```
 
+To include these nodes in path explanations, add the following to a
+configuration:
+
+```ql
+predicate includeHiddenNodes() { any() }
+```
+
 ### Unreachable nodes
 
 Consider: