Update config.rst

[meeuw]

It's required to set collections = null now

[WhyNotHugo]

What's the context for this? collections doesn't need to be null.

[meeuw]

Oh sorry for the late reply, the context is above:

Example for syncing with :storage:caldav using a null collection::

It's now required to set collections to null if you want to use null collections. It doesn't seem to be a default anymore.