testCMFCategory: update comment about categories membership to self

Add a warning about the doubt regarding the example in comment, to me
this does not depend on category being member of self.

One reason for the membership to self - in my understanding - is to make
inventory API work the same with movement to meta node categories or
movement to nodes member of this category when using
getInventory(node_category=(the meta node category))

Also remove comment about what's said to be a "mistake" ( the svn url should
be equivalent to git commit eedb2b29 )
because this was never reverted and does not seem to be a mistake, the
purpose of the change is to prevent having categories member of
themselves twice.

product/CMFCategory/tests/testCMFCategory.py

@@ -654,8 +654,9 @@ class TestCMFCategory(ERP5TypeTestCase):
       a document has destination category C and we look for all documents
       which destination is part of C category, we will not find it.
 
-      For example, the following commit was a mistake:
-    http://svn.erp5.org/erp5/trunk/products/CMFCategory/CategoryTool.py?r1=8850&r2=9997
+      ( XXX not sure of this example, I guess it works because non strict
+      membership is indexed -jerome )
+
     """
     europe = self.portal.portal_categories.resolveCategory('region/europe')
     self.assertTrue('region/europe' in europe.getCategoryList())