bpo-33649: Add a high-level section about Futures; few quick fixes (GH-9403)

Co-authored-by: Elvis Pranskevichus <elvis@magic.io>
diff --git a/Doc/library/asyncio.rst b/Doc/library/asyncio.rst
index 1511b2f..6990adb 100644
--- a/Doc/library/asyncio.rst
+++ b/Doc/library/asyncio.rst
@@ -17,6 +17,7 @@
            await asyncio.sleep(1)
            print('... World!')
 
+       # Python 3.7+
        asyncio.run(main())
 
 asyncio is a library to write **concurrent** code using