Branching related additions to docs

airflow/operators/python_operator.py

@@ -83,6 +83,8 @@ class BranchPythonOperator(PythonOperator):
     Note that using tasks with ``depends_on_past=True`` downstream from
     ``BranchPythonOperator`` is logically unsound as ``skipped`` status
     will invariably lead to block tasks that depend on their past successes.
+    ``skipped`` states propagates where all directly upstream tasks are
+    ``skipped``.
     """
     def execute(self, context):
         branch = super(BranchPythonOperator, self).execute(context)

docs/concepts.rst

@@ -211,6 +211,12 @@ is followed, and all of the other paths are skipped.
 The task_id returned by the Python function has to be referencing a task
 directly downstream from the BranchPythonOperator task.
 
+Note that using tasks with ``depends_on_past=True`` downstream from
+``BranchPythonOperator`` is logically unsound as ``skipped`` status
+will invariably lead to block tasks that depend on their past successes.
+``skipped`` states propagates where all directly upstream tasks are
+``skipped``.
+
 
 SLAs
 ''''