`._get_compiled_doc` memoizes in `._doc` yet is called in different conditions

`_generate_html_toc` adds `libmarkdown.MKD_TOC` to `.flags`, so differing from the other ones that call `_get_compiled_doc`. I am not sure what the effect of changing `.flags` is.

Since the memoized value is invalidated want to

    if self.flags &libmarkdown.MKD_TOC ==0: delattr(self, '_doc')