policy: Prefer to document python3-foo in python-foo-doc
I believe this reflects the policy that is applied in practice: at the time of writing there are 689 packages visible to my apt matching '^python-[-a-z0-9.]+-doc' and 27 matching '^python3-[-a-z0-9.]+-doc', and the wiki page https://wiki.debian.org/Python/2Removal tells us not to rename existing python-foo-doc packages to python3-doc.
Prompted by https://lists.debian.org/debian-python/2020/03/msg00071.html.