Clearer wording of os.WNOHANG documentation to avoid misinterpretation

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-11-14.21:52:21.080>
labels = ['3.7', '3.8', '3.9', 'type-feature', 'library', 'docs']
title = 'Clearer wording of os.WNOHANG documentation to avoid misinterpretation'
updated_at = <Date 2021-06-24.23:45:41.286>
user = 'https://bugs.python.org/bbmmy'

bugs.python.org fields:

activity = <Date 2021-06-24.23:45:41.286>
actor = 'iritkatriel'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Library (Lib)']
creation = <Date 2019-11-14.21:52:21.080>
creator = 'bbmmy'
dependencies = []
files = []
hgrepos = []
issue_num = 38802
keywords = []
message_count = 3.0
messages = ['356627', '356628', '396517']
nosy_count = 3.0
nosy_names = ['docs@python', 'bbmmy', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38802'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']