feat(docs): detail issue with inspect.iscoroutinefunction

Silly little PR, but I'm also opening an issue at the same time and will make an attempt to solve the problem in the next few weeks.

Author: BobTheBuidler

mypyc/doc/differences_from_python.rst

@@ -317,6 +317,7 @@ non-exhaustive list of what won't work:
 - Frames of compiled functions can't be inspected using ``inspect``
 - Compiled methods aren't considered methods by ``inspect.ismethod``
 - ``inspect.signature`` chokes on compiled functions
+- ``inspect.iscoroutinefunction`` always returns False for compiled functions, even those defined with `async def`
 
 Profiling hooks and tracing
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