Improve documentation about tzinfo.dst(None)

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-10-07.16:50:32.456>
labels = ['docs']
title = 'Improve documentation about tzinfo.dst(None)'
updated_at = <Date 2016-10-07.17:55:11.099>
user = 'https://github.com/dmoisset'

bugs.python.org fields:

activity = <Date 2016-10-07.17:55:11.099>
actor = 'belopolsky'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2016-10-07.16:50:32.456>
creator = 'Daniel Moisset'
dependencies = []
files = []
hgrepos = []
issue_num = 28386
keywords = []
message_count = 2.0
messages = ['278256', '278258']
nosy_count = 3.0
nosy_names = ['belopolsky', 'docs@python', 'Daniel Moisset']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue28386'
versions = []