Fix NeglectedSections error to not claim every section is neglected

The NeglectedSections error (the one you see if you declare a `@section` without listing the id in `@order`), looked like `vimdoc.error.NeglectedSections: Sections set(['every', 'detected', 'section']) not included in ordering.`, but some of those sections are already in `@order` and not all are actually neglected.

This fixes the error to include the neglected section names as well as the complete `@order` list.