users-guide: Fix build with sphinx 1.8
users-guide: Fix build with sphinx 1.8

It seems that both add_object_type and add_directive_to_domain both register a
directive. Previously sphinx didn't seem to mind this but as of Sphinx 1.8 it
crashes with an exception.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc