Document os.environ[x] = y and os.putenv() as thread unsafe

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-01-18.00:36:07.918>
labels = ['type-feature', '3.8', '3.9', '3.10', 'docs']
title = 'Document os.environ[x] = y and os.putenv() as thread unsafe'
updated_at = <Date 2021-03-04.18:23:52.578>
user = 'https://github.com/gpshead'

bugs.python.org fields:

activity = <Date 2021-03-04.18:23:52.578>
actor = 'eryksun'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2020-01-18.00:36:07.918>
creator = 'gregory.p.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 39375
keywords = []
message_count = 5.0
messages = ['360221', '360223', '360450', '360451', '375992']
nosy_count = 5.0
nosy_names = ['gregory.p.smith', 'docs@python', 'eryksun', 'nanjekyejoannah', 'Daniel Martin']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39375'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']