Python: Add missing QLDoc

Author: tausbn

python/ql/src/semmle/python/dataflow/new/TypeTracker.qll

@@ -12,6 +12,33 @@ class AttributeName = Internal::ContentName;
 /** Either an attribute name, or the empty string (representing no attribute). */
 class OptionalAttributeName = Internal::OptionalContentName;
 
+/**
+ * Summary of the steps needed to track a value to a given dataflow node.
+ *
+ * This can be used to track objects that implement a certain API in order to
+ * recognize calls to that API. Note that type-tracking does not by itself provide a
+ * source/sink relation, that is, it may determine that a node has a given type,
+ * but it won't determine where that type came from.
+ *
+ * It is recommended that all uses of this type are written in the following form,
+ * for tracking some type `myType`:
+ * ```ql
+ * DataFlow::LocalSourceNode myType(DataFlow::TypeTracker t) {
+ *   t.start() and
+ *   result = < source of myType >
+ *   or
+ *   exists (DataFlow::TypeTracker t2 |
+ *     result = myType(t2).track(t2, t)
+ *   )
+ * }
+ *
+ * DataFlow::LocalSourceNode myType() { myType(DataFlow::TypeTracker::end()) }
+ * ```
+ *
+ * Instead of `result = myType(t2).track(t2, t)`, you can also use the equivalent
+ * `t = t2.step(myType(t2), result)`. If you additionally want to track individual
+ * intra-procedural steps, use `t = t2.smallstep(myCallback(t2), result)`.
+ */
 class TypeTracker extends Internal::TypeTracker {
   /**
    * Holds if this is the starting point of type tracking, and the value starts in the attribute named `attrName`.