Provide mechanism for doc blocks outside vimscript files

For some plugins, there's no natural place to put doc comments like `@section` blocks to control vimdoc output, so we've resorted to a convention of adding empty `plugin/doc.vim` files purely to contain the doc comments. This is a little unintuitive and adds unnecessary runtime overhead.

We should investigate an alternative mechanism to define doc sections in a separate non-vimscript file.