Rename double specialist_document -> section
Rename double specialist_document -> section

We noticed here that `manual` is stubbed to receive a `documents`
method. In practice ManualUpdateType receives a ManualWithDocuments
instance and we should probably look to rename that to
ManualWithSections and give it a `sections` method instead.

alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher