gdb/python: Make convert_values_to_python return gdbpy_ref<>

gdb/python: Make convert_values_to_python return gdbpy_ref<>

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.