clarify concurrent.futures docs to not refer to async Futures except where truly necessary

[applio]

BPO	28052
Nosy	@applio

^{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 = None
created_at = <Date 2016-09-09.20:08:22.494>
labels = ['type-feature']
title = 'clarify concurrent.futures docs to not refer to async Futures except where truly necessary'
updated_at = <Date 2016-09-09.20:08:22.494>
user = 'https://github.com/applio'

bugs.python.org fields:

activity = <Date 2016-09-09.20:08:22.494>
actor = 'davin'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2016-09-09.20:08:22.494>
creator = 'davin'
dependencies = []
files = []
hgrepos = []
issue_num = 28052
keywords = []
message_count = 1.0
messages = ['275417']
nosy_count = 1.0
nosy_names = ['davin']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue28052'
versions = ['Python 3.5', 'Python 3.6']

[applio]

Per discussions with Yury, Lukasz and BDFL.