Clarify that __path__ can't be set to just anything

[brettcannon]

BPO	33276
Nosy	@brettcannon
PRs	bpo-33276: Clarify that __path__ can't be just any value #6554 [3.6] bpo-33276: Clarify that __path__ can't be just any value (GH-6554) #6557 [3.7] bpo-33276: Clarify that __path__ can't be just any value (GH-6554) #6556

^{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/brettcannon'
closed_at = <Date 2018-04-20.23:33:43.650>
created_at = <Date 2018-04-13.19:08:16.472>
labels = ['easy', '3.7', '3.8', 'docs']
title = "Clarify that __path__ can't be set to just anything"
updated_at = <Date 2018-04-20.23:35:47.462>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2018-04-20.23:35:47.462>
actor = 'miss-islington'
assignee = 'brett.cannon'
closed = True
closed_date = <Date 2018-04-20.23:33:43.650>
closer = 'brett.cannon'
components = ['Documentation']
creation = <Date 2018-04-13.19:08:16.472>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 33276
keywords = ['patch', 'easy']
message_count = 1.0
messages = ['315256']
nosy_count = 1.0
nosy_names = ['brett.cannon']
pr_nums = ['6554', '6557', '6556']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue33276'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[brettcannon]

https://docs.python.org/3/reference/import.html#module-path says that "regardless of its value", __path__'s mere existence is key. But then later the docs say "__path__ must be an iterable of strings, but it may be empty". Technically the latter is more accurate, so the docs should get touched up to match itself better.