Ensure tox r -e docs generates the documentation (#3047)

Author: gaborbernat

tox.ini

@@ -64,7 +64,8 @@ description = build documentation
 extras =
     docs
 commands =
-    sphinx-build -d "{envtmpdir}{/}doctree" docs "{toxworkdir}{/}docs_out" --color -b html {posargs:-b linkcheck -W}
+    {posargs: sphinx-build -d "{envtmpdir}{/}doctree" docs "{toxworkdir}{/}docs_out" --color -b linkcheck}
+    sphinx-build -d "{envtmpdir}{/}doctree" docs "{toxworkdir}{/}docs_out" --color -b html -W
     python -c 'print(r"documentation available under file://{toxworkdir}{/}docs_out{/}index.html")'
 
 [testenv:pkg_meta]