CField.size from the ctypes module does not behave as documented on bitfields

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-06-07.03:46:36.822>
labels = ['ctypes', 'type-bug', 'docs']
title = 'CField.size from the ctypes module does not behave as documented on bitfields'
updated_at = <Date 2019-07-08.17:32:40.963>
user = 'https://bugs.python.org/EricWieser'

bugs.python.org fields:

activity = <Date 2019-07-08.17:32:40.963>
actor = 'shihai1991'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'ctypes']
creation = <Date 2019-06-07.03:46:36.822>
creator = 'Eric Wieser'
dependencies = []
files = []
hgrepos = []
issue_num = 37187
keywords = ['patch']
message_count = 2.0
messages = ['344895', '344921']
nosy_count = 7.0
nosy_names = ['amaury.forgeotdarc', 'belopolsky', 'meador.inge', 'docs@python', 'Eric Wieser', 'mangrisano', 'shihai1991']
pr_nums = ['14647']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37187'
versions = []