Added limitations from wiki.

docs/src/userguide/limitations.rst

@@ -14,3 +14,59 @@ future version 1.0 of Cython is planned to provide full Python
 language compatibility.
 
 .. _`list of remaining compatibility issues`: http://trac.cython.org/cython_trac/query?status=assigned&status=new&status=reopened&component=Python+Semantics&component=Python3+Semantics&order=priority&col=id&col=summary&col=component&col=status&col=type&col=priority&col=milestone
+
+Below is a list of differences that we will probably not be addressing.
+Most of these things that fall more into the implementation details rather
+than semantics, and we may decide not to fix (or require a --pedantic flag to get).
+
+
+==========
+Nested tuple argument unpacking.
+==========
+
+::
+
+    def f((a,b), c):
+        pass
+
+This was removed in Python 3.
+
+
+==========
+Inspect support
+==========
+
+While it is quite possible to emulate the interface of functions in
+Cython's own function type, and recent Cython releases have seen several
+improvements here, the "inspect" module does not consider a Cython
+implemented function a "function", because it tests the object type
+explicitly instead of comparing an abstract interface or an abstract
+base class. This has a negative impact on code that uses inspect to
+inspect function objects, but would require a change to Python itself.
+
+
+==========
+Stack frames
+==========
+
+Currently we generate fake tracebacks as part of exception propagation,
+but don't fill in locals and can't fill in co_code.
+To be fully compatible, we would have to generate these stack frame objects at
+function call time (with a potential performance penalty).  We may have an
+option to enable this for debugging.
+
+==========
+Identity vs. equality for inferred literals.
+==========
+
+::
+    a = 1.0        # a inferred to be double
+    b = c = None   # a inferred to be type object
+    if some_runtime_expression:
+        b = a
+        c = a
+    print b is c   # py float created twice
+
+
+
+