Add reference to subinterpreters.

Author: anordin95

Doc/howto/a-conceptual-overview-of-asyncio.rst

@@ -822,7 +822,7 @@ instruction is executed at a time.
 Of course, since processes are generally entirely independent from one another,
 the GIL in one process won't be impeded by the GIL in another process.
 Granted, I believe there are ways to also get around the GIL in a single process
-by leveraging C extensions.
+by leveraging C extensions or via subinterpreters.
 
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 multithreading & asyncio