ConfigParser.set() docs should mention allow_no_value

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-06-28.09:23:20.750>
labels = ['3.8', 'type-feature', '3.7', '3.9', 'docs']
title = 'ConfigParser.set() docs should mention allow_no_value'
updated_at = <Date 2019-08-07.12:56:36.579>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2019-08-07.12:56:36.579>
actor = 'nanjekyejoannah'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2019-06-28.09:23:20.750>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 37438
keywords = ['patch']
message_count = 1.0
messages = ['346813']
nosy_count = 3.0
nosy_names = ['ncoghlan', 'docs@python', 'nanjekyejoannah']
pr_nums = ['14455']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue37438'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']