C++: Add a comment on the old SSA library.

cpp/ql/lib/semmle/code/cpp/controlflow/SSA.qll

@@ -15,6 +15,13 @@ class StandardSsa extends SsaHelper {
 }
 
 /**
+ * NOTE: If possible, prefer the SSA classes exposed by the new dataflow
+ * library:
+ * ```
+ * import semmle.code.cpp.dataflow.new.DataFlow
+ * // use `DataFlow::Definition`
+ * ```
+ *
  * A definition of one or more SSA variables, including phi node definitions.
  * An _SSA variable_, as defined in the literature, is effectively the pair of
  * an `SsaDefinition d` and a `StackVariable v`, written `(d, v)` in this