Document behavior when CDLL is called with None as an argument on POSIX systems

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-06-16.04:38:10.383>
labels = ['ctypes', 'type-feature', 'docs']
title = 'Document behavior when CDLL is called  with None as an argument on POSIX systems'
updated_at = <Date 2016-06-16.04:48:22.898>
user = 'https://bugs.python.org/JeffreyEsquivelSibaja'

bugs.python.org fields:

activity = <Date 2016-06-16.04:48:22.898>
actor = 'martin.panter'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'ctypes']
creation = <Date 2016-06-16.04:38:10.383>
creator = 'Jeffrey Esquivel Sibaja'
dependencies = []
files = []
hgrepos = []
issue_num = 27329
keywords = []
message_count = 1.0
messages = ['268641']
nosy_count = 2.0
nosy_names = ['docs@python', 'Jeffrey Esquivel Sibaja']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue27329'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']