bpo-38260: Add Docs on asyncio.run (GH-16337)

Add docs about return and raise exception on asyncio.run





https://bugs.python.org/issue38260



Automerge-Triggered-By: @asvetlov

Lib/asyncio/runners.py

@@ -21,6 +21,10 @@ def run(main, *, debug=False):
     It should be used as a main entry point for asyncio programs, and should
     ideally only be called once.
 
+    Return a result of *coro* execution, or raise a RuntimeError
+    if `asyncio.run()`is called from a running event loop, or a ValueError
+    if `main` is not a courutine.
+
     Example:
 
         async def main():