Special interaction with /etc/NetworkManager/NetworkManager.conf is not documented

The auto-up/down behaviour is modified depending on the contents of /etc/NetworkManager/NetworkManager.conf. This was done in 0.6.8ubuntu10:

ifupdown (0.6.8ubuntu10) intrepid; urgency=low
  https://lists.ubuntu.com/archives/ubuntu-devel/2008-October/026656.html:
  * stop ifupdown to auto-up/down interfaces if network-manager's
    ifupdown plugin is in managed=true mode
    [...]
 -- Alexander Sack <email address hidden> Thu, 09 Oct 2008 16:46:47 +0200

This is an unusual interaction and should be documented in interfaces(5) at a minimum, and perhaps ifup(8) as well.

I think this is important as the documentation for ifupdown is quite comprehensive, and this behaviour is quite special and unexpected to anybody debugging problems with their network configuration. Example: http://askubuntu.com/a/304291/7808 and bug 1187693.

We should not carry such an unusual delta over Debian without making sure it is documented in the places that people look to explain unexpected behaviour.