Add comment about recent workaround for circular import dependency

See commit a93ea5f1.

product/CMFActivity/ActivityTool.py

@@ -603,6 +603,9 @@ def cancelProcessShutdown():
   is_running_lock.release()
   has_processed_shutdown = False
 
+# Due to a circular import dependency between this module and
+# Products.ERP5Type.Core.Folder, both modules must import after the definitions
+# of getCurrentNode and Folder (the later is a base class of BaseTool).
 from Products.ERP5Type.Tool.BaseTool import BaseTool
 # Activating a path means we tried to avoid loading useless
 # data in cache so there would be no gain to expect.

product/ERP5Type/Core/Folder.py

@@ -1665,6 +1665,7 @@ class Folder(OFSFolder2, CMFBTreeFolder, CMFHBTreeFolder, Base, FolderMixIn):
         pass
     return '%s/%s' % (url, icon)
 
+# See comment in Products.CMFActivity.ActivityTool about the import of BaseTool.
 from Products.CMFActivity.ActivityTool import getCurrentNode
 
 # We browse all used class from btree and hbtree and set not implemented