Allow grouping functions/flags/etc into subsections

For plugins with a lot of functions or flags, there doesn't seem to be a directive to tell vimdoc to group item docs into a subsection.

For instance if I wanted all the autoload functions that start with `maktaba#ensure#`[1] to go into a subsection about those assertions instead of into the big list of all maktaba-functions, I don't see a way to configure vimdoc to do that.

[1]: https://github.com/google/vim-maktaba/blob/b31525d7f6170c68f900abe24748ac9fbaf1ee4d/doc/maktaba.txt#L684