Fix asyncio.get_event_loop() documentation (GH-18051)

Mention that the function implicitly creates new event loop only if called from the main thread.
diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index ee995e0..25a3692 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -38,8 +38,10 @@
 
 .. function:: get_event_loop()
 
-   Get the current event loop.  If there is no current event loop set
-   in the current OS thread and :func:`set_event_loop` has not yet
+   Get the current event loop.
+
+   If there is no current event loop set in the current OS thread,
+   the OS thread is main, and :func:`set_event_loop` has not yet
    been called, asyncio will create a new event loop and set it as the
    current one.