English doc: Introduce `@endmorerefs`

@morerefs now starts a ragged-right environment, which improves formatting in PDF output.

The only interesting changes are in files en/macros.texi, modifying @morerefs and introducing @endmorerefs, and in file en/contributor/doc-work.itexi, documenting the change. All other files contain mechanical additions of @endmorerefs at the appropriate places.

Note that the Catalan and Hungarian manuals are not completely translated, thus using files from the English version that contain @endmorerefs. For those two languages, @endmorerefs has been defined as a no-op currently.