Add explanatory comment.

gui/qt/main_window.py

@@ -163,6 +163,10 @@ class ElectrumWindow(QMainWindow, PrintError):
             self.connect(self, QtCore.SIGNAL('network'), self.on_network_qt)
             interests = ['updated', 'new_transaction', 'status',
                          'banner', 'verified']
+            # To avoid leaking references to "self" that prevent the
+            # window from being GC-ed when closed, callbacks should be
+            # methods of this class only, and specifically not be
+            # partials, lambdas or methods of subobjects.  Hence...
             self.network.register_callback(self.on_network, interests)
             # set initial message
             self.console.showMessage(self.network.banner)