Document that os.fork is not allowed in subinterpreters

BPO	38778
Nosy	@ericsnowcurrently, @phmc, @miss-islington
PRs	bpo-38778: Document that os.fork is not allowed in subinterpreters #17123 [3.8] bpo-38778: Document that os.fork is not allowed in subinterpreters (GH-17123) #17179

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-11-15.21:39:07.153>
created_at = <Date 2019-11-12.14:38:20.507>
labels = ['3.8', '3.9', 'docs']
title = 'Document that os.fork is not allowed in subinterpreters'
updated_at = <Date 2019-11-15.21:39:07.152>
user = 'https://github.com/phmc'

bugs.python.org fields:

activity = <Date 2019-11-15.21:39:07.152>
actor = 'eric.snow'
assignee = 'docs@python'
closed = True
closed_date = <Date 2019-11-15.21:39:07.153>
closer = 'eric.snow'
components = ['Documentation']
creation = <Date 2019-11-12.14:38:20.507>
creator = 'pconnell'
dependencies = []
files = []
hgrepos = []
issue_num = 38778
keywords = ['patch']
message_count = 5.0
messages = ['356459', '356693', '356704', '356719', '356721']
nosy_count = 4.0
nosy_names = ['docs@python', 'eric.snow', 'pconnell', 'miss-islington']
pr_nums = ['17123', '17179']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue38778'
versions = ['Python 3.8', 'Python 3.9']