Issue #3080: Document the name attribute of the _inittab structure

The name is an ASCII encoded string.
diff --git a/Include/import.h b/Include/import.h
index 710fd22..6a62c94 100644
--- a/Include/import.h
+++ b/Include/import.h
@@ -87,7 +87,7 @@
 PyAPI_FUNC(int)_PyImport_FixupExtensionObject(PyObject*, PyObject *, PyObject *);
 
 struct _inittab {
-    char *name;
+    char *name;                 /* ASCII encoded string */
     PyObject* (*initfunc)(void);
 };
 PyAPI_DATA(struct _inittab *) PyImport_Inittab;