Fix get_event_loop documentation

[asvetlov]

BPO	39381
Nosy	@asvetlov, @1st1
PRs	bpo-39381: Fix asyncio.get_event_loop() documentation #18051 [3.8] bpo-39381: Fix asyncio.get_event_loop() documentation (GH-18051) #18084 [3.7] bpo-39381: Fix asyncio.get_event_loop() documentation (GH-18051) #18085

^{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 2020-01-20.22:52:57.546>
created_at = <Date 2020-01-18.13:30:41.363>
labels = ['3.7', '3.8', '3.9', 'expert-asyncio']
title = 'Fix get_event_loop documentation'
updated_at = <Date 2020-01-20.22:52:57.545>
user = 'https://github.com/asvetlov'

bugs.python.org fields:

activity = <Date 2020-01-20.22:52:57.545>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-20.22:52:57.546>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2020-01-18.13:30:41.363>
creator = 'asvetlov'
dependencies = []
files = []
hgrepos = []
issue_num = 39381
keywords = ['patch']
message_count = 1.0
messages = ['360244']
nosy_count = 2.0
nosy_names = ['asvetlov', 'yselivanov']
pr_nums = ['18051', '18084', '18085']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39381'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[asvetlov]

The current documentation says: "If there is no current event loop set in the current OS thread and set_event_loop() has not yet been called, asyncio will create a new event loop and set it as the current one."

https://docs.python.org/3.7/library/asyncio-eventloop.html#asyncio.get_event_loop

This is not correct, a new loop is created implicitly only for the main thread, all other threads require set_event_loop() call