users_guide: fix sphinx error caused by non-explicit override

Authored by watashi on Sep 20 2018, 3:05 PM.

Description

users_guide: fix sphinx error caused by non-explicit override

Encouter following error when make:

Extension error:
The 'ghc-flag' directive is already registered to domain std

as we register ghc-flag to std in add_object_type first and then
overtride it in add_directive_to_domain.

Test Plan:

make -C utils/haddock/doc html SPHINX_BUILD=/usr/bin/sphinx-build

Reviewers: austin, bgamari, patrickdoc

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5161

(cherry picked from commit 8c7d33a8ff6d3ea55b5dc8108d9441521af68ab8)