Run documentation lints with two jobs.

tox.ini

@@ -56,15 +56,15 @@ basepython =
 setenv =
     PYTHONHASHSEED = 100
 commands =
-    sphinx-build -b apicheck -d {envtmpdir}/doctrees docs docs/_build/apicheck
+    sphinx-build -j2 -b apicheck -d {envtmpdir}/doctrees docs docs/_build/apicheck
 
 [testenv:configcheck]
 commands =
-    sphinx-build -b configcheck -d {envtmpdir}/doctrees docs docs/_build/configcheck
+    sphinx-build -j2 -b configcheck -d {envtmpdir}/doctrees docs docs/_build/configcheck
 
 [testenv:linkcheck]
 commands =
-    sphinx-build -W -b linkcheck -d {envtmpdir}/doctrees docs docs/_build/linkcheck
+    sphinx-build -j2 -b linkcheck -d {envtmpdir}/doctrees docs docs/_build/linkcheck
 
 [testenv:flake8]
 commands =