Allow grouping functions/flags/etc into subsections

[dbarnett]

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.