Document __warningregistry__

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-10-10.01:50:43.793>
labels = ['type-bug', 'library']
title = 'Document __warningregistry__'
updated_at = <Date 2018-10-12.16:23:11.700>
user = 'https://github.com/stevendaprano'

bugs.python.org fields:

activity = <Date 2018-10-12.16:23:11.700>
actor = 'eric.araujo'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-10-10.01:50:43.793>
creator = 'steven.daprano'
dependencies = []
files = []
hgrepos = []
issue_num = 34948
keywords = []
message_count = 4.0
messages = ['327449', '327454', '327463', '327474']
nosy_count = 2.0
nosy_names = ['steven.daprano', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34948'
versions = []