Document how collections.ChainMap() determines iteration order

BPO	36060
Nosy	@rhettinger
PRs	bpo-36060: Document how collections.ChainMap() determines iteration order #11969 [3.7] bpo-36060: Document how collections.ChainMap() determines iteration order (GH-11969) #11978

^{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 = 'https://github.com/rhettinger'
closed_at = <Date 2019-02-21.17:17:08.920>
created_at = <Date 2019-02-21.00:30:45.434>
labels = ['3.7', '3.8', 'docs']
title = 'Document how collections.ChainMap() determines iteration order'
updated_at = <Date 2019-02-21.17:47:48.485>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2019-02-21.17:47:48.485>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2019-02-21.17:17:08.920>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2019-02-21.00:30:45.434>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 36060
keywords = ['patch']
message_count = 3.0
messages = ['336165', '336239', '336244']
nosy_count = 1.0
nosy_names = ['rhettinger']
pr_nums = ['11969', '11978']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue36060'
versions = ['Python 3.7', 'Python 3.8']