Make convert_values_to_python return a gdbpy_ref<> directly rather
than building a gdbpy_ref<>, releasing it, and then having a new
gdbpy_ref<> created to hold the result.
I also added a header comment to convert_values_to_python.
gdb/ChangeLog:
* python/py-function.c (convert_values_to_python): Return
gdbpy_ref<>. Add header comment.
(fnpy_call): Adjust.