add documentation

doc/development/implementation/timemachine.rst

@@ -10,6 +10,8 @@ product can be bought, you should use :py:meth:`time_machine_now`.
 
 .. autofunction:: pretix.base.timemachine.time_machine_now
 
+.. autofunction:: pretix.base.timemachine.time_machine_now_assigned
+
 Background tasks
 ----------------
 

src/pretix/base/timemachine.py

@@ -60,6 +60,11 @@ def time_machine_now(default=False):
 
 @contextmanager
 def time_machine_now_assigned(now_dt):
+    """
+    Use this context manager to assign current datetime for time machine mode. Useful e.g. for background tasks.
+
+    :param now_dt: The datetime value to assign. May be `None` to disable time machine.
+    """
     try:
         timemachine_now_var.set(now_dt)
         yield