Document that subprocess.Popen does not set PWD

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-06-29.02:32:34.277>
labels = ['type-feature', 'docs']
title = 'Document that subprocess.Popen does not set PWD'
updated_at = <Date 2017-07-02.00:38:27.753>
user = 'https://bugs.python.org/jamesdlin'

bugs.python.org fields:

activity = <Date 2017-07-02.00:38:27.753>
actor = 'jamesdlin'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2017-06-29.02:32:34.277>
creator = 'jamesdlin'
dependencies = []
files = []
hgrepos = []
issue_num = 30798
keywords = []
message_count = 3.0
messages = ['297244', '297499', '297500']
nosy_count = 3.0
nosy_names = ['r.david.murray', 'docs@python', 'jamesdlin']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30798'
versions = ['Python 3.6']