Preparing for 1.2 release

actually no, cmf 1.1+ dropped the symfony 2.2 support so this is only relevant for cmf 1.0. which probably means we could drop the documentation of this tag.

if we do what we promised in the `.. caution::`, we actually should remove the tag from core bundle alltogether. in the interest of BC i think we should leave it in until 2.0 - but not have any documentation anymore, its just legacy.