Multiprocessing docs don't describe thread-safety

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-05-29.13:25:00.812>
labels = ['docs']
title = "Multiprocessing docs don't describe thread-safety"
updated_at = <Date 2020-08-17.18:29:13.905>
user = 'https://github.com/bensimner'

bugs.python.org fields:

activity = <Date 2020-08-17.18:29:13.905>
actor = 'iritkatriel'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2020-05-29.13:25:00.812>
creator = 'bjs'
dependencies = []
files = []
hgrepos = []
issue_num = 40815
keywords = []
message_count = 3.0
messages = ['370310', '375103', '375563']
nosy_count = 4.0
nosy_names = ['docs@python', 'alan.briolat', 'bjs', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue40815'
versions = []