[Python] Fix docstrings for %callback functions

Reinstates autodoc for callback function testcase from #467, actually
tests the resulting docstring in the _runme.py and fixes SWIG/Python
so the expected result is obtained.

Examples/test-suite/autodoc.i

@@ -114,6 +114,12 @@
   }
 %}
 
+%callback("%(uppercase)s_CALLBACK") func_cb;
+
+%inline {
+  int func_cb(int c, int d) { return c; }
+}
+
 // Bug 3310528
 %feature("autodoc","1") banana; // names + types
 %inline %{