Add warning about unregistering image formats.

Add warning to documentation to warn that unregistering image formats
that are being used will create an error when viewing or editing pages.

See https://groups.google.com/forum/#!topic/wagtail/X8xTUs-2npA and
https://github.com/wagtail/wagtail/issues/1471#issuecomment-118436706.

docs/advanced_topics/customisation/page_editing_interface.rst

@@ -143,6 +143,9 @@ To begin, import the ``Format`` class, ``register_image_format`` function, and o
 
 To unregister, call ``unregister_image_format`` with the string of the ``name`` of the ``Format`` as the only argument.
 
+  .. warning::
+     Unregistering ``Format`` objects will cause errors viewing or editing pages that reference them.
+
 .. _custom_edit_handler_forms:
 
 Customising generated forms