[mypyc] feat(docs): detail issue with inspect.iscoroutinefunction (#19309)

Silly little PR, but this note could have saved me some debugging time.

I also opened [an issue](mypyc/mypyc#1110) and
will make an attempt to solve the problem sometime in Q3.

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`` and ``asyncio.iscoroutinefunction`` will always return False for compiled functions, even those defined with `async def`
 
 Profiling hooks and tracing
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