Write docstring to fixtures

[jgomo3]

This is what I get when running pytest --fixtures:

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django_db_modify_db_settings_xdist_suffix
    ...python3.5/site-packages/pytest_django/fixtures.py:23: no docstring available
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django_db_modify_db_settings
    ...python3.5/site-packages/pytest_django/fixtures.py:51: no docstring available
django_db_use_migrations
   ...python3.5/site-packages/pytest_django/fixtures.py:56: no docstring available
django_db_keepdb
    ...python3.5/site-packages/pytest_django/fixtures.py:61: no docstring available
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mailoutbox
    ...python3.5/site-packages/pytest_django/plugin.py:428: no docstring available
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