Python: add missing qldoc

More doc is needed, but this should turn the tests green

Author: yoff

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

@@ -129,9 +129,11 @@ newtype TNode =
     f = any(VariableCapture::CapturedVariable v).getACapturingScope() and
     exists(TFunction(f))
   } or
+  /** A synthetic node representing the values of variables captured by a comprehension. */
   TSynthCompCapturedVariablesArgumentNode(Comp comp) {
     comp.getFunction() = any(VariableCapture::CapturedVariable v).getACapturingScope()
   } or
+  /** A synthetic node representing the values of variables captured by a comprehension after the output has been computed. */
   TSynthCompCapturedVariablesArgumentPostUpdateNode(Comp comp) {
     comp.getFunction() = any(VariableCapture::CapturedVariable v).getACapturingScope()
   } or