Fix internal references in docs/as-module

Further, document all attributes that are referred to, and ensure that there
will be no broken references again by letting Travis call Sphinx with -W -n.

.travis.yml

@@ -9,7 +9,7 @@ install:
  - pip install -r docs/requirements.txt
 script:
  - python3 -m pylint -r n -d bad-whitespace,missing-docstring,too-many-arguments,locally-disabled,line-too-long,too-many-public-methods,too-many-lines,too-many-instance-attributes,too-many-locals,too-many-branches,too-many-statements,inconsistent-return-statements,invalid-name,wildcard-import,unused-wildcard-import instaloader
- - make -C docs html
+ - make -C docs html SPHINXOPTS="-W -n"
 deploy:
   - provider: pypi
     user: aandergr