Docstring for Application._invalidate_handler.

Author: jonathanslenders

prompt_toolkit/application/application.py

@@ -474,7 +474,11 @@ def gather_events():
 
  def _invalidate_handler(self, sender):
  """
- Handler to allow invalidate to be called with the correct signature.
+ Handler for invalidate events coming from UIControls.
+
+ (This handles the difference in signature between event handler and
+ `self.invalidate`. It also needs to be a method -not a nested
+ function-, so that we can remove it again .)
  """
  self.invalidate()