Python: Update comment.

Author: yoff

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -15,13 +15,11 @@ private import semmle.python.frameworks.data.ModelsAsData
 /**
  * IPA type for data flow nodes.
  *
- * Flow between SSA variables are computed in `Essa.qll`
+ * Nodes broadly fall into three categories.
  *
- * Flow from SSA variables to control flow nodes are generally via uses.
- *
- * Flow from control flow nodes to SSA variables are generally via assignments.
- *
- * The current implementation of these cross flows can be seen in `EssaTaintTracking`.
+ * - Control flow nodes: Flow between these is based on use-use flow computed via an SSA analysis.
+ * - Module variable nodes: These represent global variables and act as canonical targets for reads and writes of these.
+ * - Synthetic nodes: These handle flow in various special cases.
  */
 newtype TNode =
   /** A node corresponding to a control flow node. */