Add comment to explain why "better" GIL handling is not needed in special...

Add comment to explain why "better" GIL handling is not needed in special methods (because it's held anyway).

Cython/Compiler/Nodes.py

@@ -2184,7 +2184,7 @@ class FuncDefNode(StatNode, BlockNode):
         if self.entry.is_special and self.entry.name == "__hash__":
             # Returning -1 for __hash__ is supposed to signal an error
             # We do as Python instances and coerce -1 into -2.
-            assure_gil('success')
+            assure_gil('success')  # in special methods, the GIL is owned anyway
             code.putln("if (unlikely(%s == -1) && !PyErr_Occurred()) %s = -2;" % (
                 Naming.retval_cname, Naming.retval_cname))